This is only a preview of the May 2009 issue of Silicon Chip. You can view 31 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Dead-Accurate 6-Digit GPS-Locked Clock, Pt.1":
Items relevant to "230VAC 10A Full-Wave Motor Speed Controller":
Items relevant to "Precision 10V DC Reference For Checking DMMs":
Items relevant to "Input Attenuator For The Digital Audio Millivoltmeter":
Purchase a printed copy of this issue for $10.00. |
By JIM ROWE
Precision
10V DC reference
for checking DMMs
Have you ever checked the calibration of your digital multimeter?
Yeah, we know – you haven’t because there is no easy (read
cheap) way of doing it. But now you can with this precision DC
voltage reference that can be built in a few hours. Without any
need for adjustment it will provide you with a 10.000V DC source
accurate to within ±3mV, ie, an accuracy of ±0.03%.
M
OST OF US DON’T ever get our
DMMs calibrated, though they
do drift out of calibration over years
of use. If you are using them in your
occupation, they should be checked
every year or so – otherwise how can
you trust the readings?
But it can cost quite a bit to send
a DMM away to a standards lab for
calibration – more than many DMMs
are worth. So generally we either hope
for the best or simply buy a new DMM
if we suspect that our existing meter
has drifted too far out of calibration.
Back in the 1970s when DMMs first
became available, the only practical
62 Silicon Chip
source of an accurately known DC voltage was the Weston cell, a wet chemical “primary cell” which had been
developed in 1893 and had become
the international standard for EMF/
voltage in 1911 (see panel). It produced
an accurate 1.0183V reference which
could be used to calibrate DMMs and
other instruments.
Unfortunately, Weston cells were
fairly expensive and few people had
direct access to one for meter calibration. As a result, most people tended to
use a reasonably fresh mercury cell as
a “poor man’s” voltage reference. Fresh
mercury cells have a terminal voltage
very close to 1.3566V at 20°C and this
falls quite slowly to about 1.3524V
after a year or so. Silver oxide cells
can also used for the same purpose,
having a stable terminal voltage very
close to 1.55V.
Of course, batteries of any kind have
a tendency to obey Murphy’s Law
and usually turn out to have quietly
expired before you need them. And
although mercury and silver oxide
cells have quite a long life, especially
if you use them purely as a voltage
reference, they certainly aren’t immune to this problem. Which means
that these batteries make a pretty flaky
siliconchip.com.au
voltage reference, at best.
Fortunately, in the 1980s semiconductor makers developed a relatively low-cost source of stable and
accurately predictable DC voltage: the
precision monolithic voltage reference
(PMVR). This is a kind of up-market
relative of the familiar 3-terminal
voltage regulator IC. Like 3-terminal
regulators, PMVRs produce a regulated
DC output voltage when they are fed
with unregulated DC power.
The Analog Devices AD588 device
we’re using in this new voltage reference project incorporates a number
of recent advances in PMVR technology. These include an ion-implanted
“buried” zener reference diode plus
high-stability thin-film resistors on
the wafer, which are laser trimmed
to minimise drift and provide high
initial accuracy.
It also incorporates highly stable
on-chip op amps which are configured
to allow Kelvin connections to the
load and/or external current boosters,
for driving long lines or high current
loads.
Block diagram
You can see what’s inside the AD588
in the block diagram of Fig.1. The
voltage reference cell is at upper left,
consisting of the “buried” zener (6.5V)
and its current source together with op
amp A1. All the resistors (R1-R6) are
high-stability thin-film resistors, laser
trimmed to allow the gain of A1 to be
set to a high degree of precision – so
the cell’s basic output voltage (between
pins VHI and VLO) is initially set to
10.000V ±3mV (for the AD588JQ/AQ
version we use here), without any
external adjustment.
Temperature compensation inside
the cell also gives the basic voltage
reference a very low temperature drift
coefficient: typically ±2ppm/°C. In
addition, resistors R4 and R5 can be
configured to provide a very accurate
“centre tap” for this voltage, allowing
the chip to be used as a precise source
of ±5.000V ±1.5mV.
Although this basic untrimmed initial accuracy of the AD588 (10.000V
±0.03%) is quite good enough for
calibrating the majority of low-cost
DMMs, the chip can also be trimmed
very easily to improve its accuracy by
a factor of greater than 10 times – ie, to
an accuracy around ±0.002%.
This is done by connecting the GAIN
ADJ pin to a 100kΩ trimpot, connected
siliconchip.com.au
VHI
+Vs
(2)
A3 IN
(6)
(4)
NOISE
REDUCTION
(3)
A3
SENSE
(7)
RB
A3
6.5V
GAIN
ADJ
(5)
A1
R3
(1) A3
OUT
R1
R4
R2
R5
A4
GND
SENSE+
R6
(9)
(15) A4
OUT
(14) A4
A2
SENSE
GND (10)
SENSE–
(16)
–Vs
(8)
VLO
(12)
BAL
ADJ
(11)
VCT
(13)
A4 IN
Fig.1: block diagram of the AD588 voltage reference. It contains four op
amps (A1-A4) plus a “buried” zener and its current source to provide the
voltage reference cell.
Specifications
Output voltage: 10.000V DC
Sensing: internal or remote sensing to compensate for output cable
voltage drop
Basic accuracy: ±0.03% (±3mV) without adjustment, ±0.002% after
optional trim adjustment and calibration
Long term drift: <15ppm per 1000 hours, mostly in first year of operation
Temperature coefficient: 3ppm/°C between -25°C and +85°C
Maximum output current: 10mA
Noise on output: less than 6mV RMS
Load regulation: less than ±50μV/mA for loads up to 10mA
Supply line regulation: less than 200μV/V
Power supply: 12V AC, current drain <60mA
between the VHI and VLO terminals.
The pot allows the gain of A1 to be
adjusted for a very small output voltage range (approximately -3.5mV to
+7.5mV) without any adverse effect
on temperature stability. Of course,
in order to take advantage of this trimming feature, you must have access
to an even higher precision voltage
reference, to compare it with.
Op amp A2 is used to allow accurate
“ground sensing”, ensuring that the
external system ground is accurately
held at the same potential as VLO.
And op amps A3 and A4, which are
internally compensated, are provided
to act as output buffers for the VHI and
VLO voltages, as well as providing for a
full Kelvin (ie, remote sensing) output
connection.
The full circuit
As you can see from the circuit
schematic of Fig.2, there’s not a great
deal in our new precision voltage
reference apart from the all-important
AD588 chip (IC1). This does just about
everything.
All that we need to do in the rest
of the circuit is provide it with a
moderately regulated and filtered
power source of ±15V and also make
its buffered output voltage available,
either at the main local terminals or at
May 2009 63
68
+15V
A
2200 F
25V
K
K
ZD1
15V
1W
A3 OUT
A3 IN–
D1
1.8k
11
A
12V AC
INPUT
2
+Vs
470 F 100nF
16V
A
22
12 BAL
POWER
LED1
ADJ
K
7
CON1
K
A3 IN+
VHI
IC1
AD588AQ
(OR JQ)
VLO
A4 IN+
9
A
GAIN
ADJ
GND
SENSE–
NR
680nF
MKT
1.8k
D2
VCT
A4 IN–
GND
SENSE+
A4 OUT
1
+10.00V
OUT
3
S1a
4
6
TRIM
VR1*
100k
25T
5
10
+10.00V
SENSE
INT/EXT
SENSING
8
0V SENSE
13
14
S1b
15
0V OUT
–Vs
68
–15V
A
2200 F
25V
K
ZD2
15V
1W
470 F
100nF
16V
16
* TRIMPOT OPTIONAL
(SEE TEXT)
D1, D2: 1N4004
A
SC
2009
PRECISION DC VOLTAGE REFERENCE
ZD1, ZD2
A
LED
K
K
K
A
Fig.2: the circuit uses the AD588 precision voltage reference (IC1) and not much else. Diodes D1 & D2 function as halfwave rectifiers and feed zener diodes ZD1 & ZD2 to provide ±15V rails for the IC.
the end of a cable connecting a remote
load to them.
The power supply configuration is
quite straightforward and uses a halfwave rectifier circuit to produce each
DC supply rail from a 12V AC input
(which can be a 12V 500mA AC plugpack). Diodes D1 & D2 form the rectifiers, with filtering provided by two
2200μF electrolytic capacitors. Zener
diodes ZD1 and ZD2 (both 15V types)
then provide moderate regulation for
the two supply rails, in conjunction
with the two 68Ω series resistors.
A small amount of additional filtering is provided by two 470μF electrolytics, together with 100nF bypass
capacitors at the supply pins for IC1.
Note that power indicator LED1 is
connected directly between the two
supply rails, in series with two 1.8kΩ
current-limiting resistors.
What This Voltage Reference Cannot Do
While this 10V DC reference is very
handy if you want to check the DC accuracy of your digital multimeter, it cannot tell
you anything about your DMM’s accuracy
in its other modes such as DC and AC
current, AC voltage and resistance. So just
because you have done a simple check
on the DC voltage accuracy, don’t let it
lull you into a false sense of security that
everything is well with your DMM.
In fact, it is possible that this 10V DC
reference may alert you to the fact that
your DMM has drifted well away from its
initial calibration which may have been
pretty good when you purchased it. How
many years ago was that?
64 Silicon Chip
If you are using DMMs in your occupation, they should be calibrated every year
or so, otherwise you cannot really trust the
readings. Moreover, if you have dropped
your multimeter, it definitely should be suspected, particularly if its internal calibration
is performed by tweaking potentiometers.
Let’s face it, most DMMs get dropped from
to time – that’s just normal.
If you need a full performance verification of all functions and ranges for your
work then that is best performed by an
accredited calibration laboratory.
For information and a quote on DMM
calibration, contact Trio-Smartcal on 1300
134 091. www.triosmartcal.com.au
The connections for IC1 itself are
fairly easy to follow. The 680nF capacitor connected to ground from the
NR pin (7) is included to provide additional low-pass filtering of any noise
generated by the AD588’s buried zener.
It works in conjunction with series
resistor RB, as shown in Fig.1.
Op amp A3 inside IC1 is used as the
positive voltage output buffer, with its
non-inverting input (pin 4) connected
directly to VHI (pin 6). The inverting
input (pin 3) is brought out to the
external positive sense terminal (for
remote sensing) and also to S1a, which
allows it to be connected directly to
the positive output (pin 1) for local
sensing.
Op amp A4 is connected in the
same way, as the negative output
voltage buffer. Here, the op amp’s
non-inverting input (pin 13) is connected directly to the reference cell’s
VLO output (pin 8), while the inverting
input (pin 14) is brought out to the
negative sense terminal for remote
sensing and also to S1b, to connect it
directly to the negative output (pin 15)
for local sensing.
So what is the purpose of the
“optional” trimpot VR1? That is for
trimming the AD588’s output voltage
siliconchip.com.au
15V
+
FER V CD
V51+
+10V OUT
100k
IC1 AD588
TRIM
INT/EXT SENSING
680nF
V 5 115V
68
2200F 25V
S1
+
ZD2
470 F
100nF
D2
4004
D1
4004
LED1
PWR
1.8k
1 diecast aluminium box, 111 x
60 x 54mm (Jaycar HB-5063
or similar)
1 PC board, code 04305091, 84
x 53.5mm
1 DPDT on-on mini toggle switch
(S1)
1 2.5mm PC-mount DC power
socket (CON1)
1 16-pin machined IC socket
2 binding post terminals, red
2 binding post terminals, black
4 M3 x 25mm tapped metal
spacers
4 M3 x 6mm screws, countersink
head
4 M3 x 6mm screws, pan head
1 100kΩ 25T top adjust trimpot
(optional – see text)
1 150mm length blue hookup
wire
1 150mm length red hookup wire
1 100mm length 0.7mm tinned
copper wire
N OI SI C E R P
VR1
1.8k
Parts List
+10V SENSE
470 F
100nF
25V
22
CON1
12V AC INPUT
1 92200
0 5 0 3 40F
9002 ©
68
ZD1
0V OUT
0V SENSE
Fig.3: follow this diagram to install the parts on the PC board and
complete the external wiring. Note that switch S1 actually mounts
on the lid of the case and not directly on the PC board – see text.
This view shows
the completed
PC board with
the optional
trimpot (VR1)
in position.
Install VR1 only
if you intend
calibrating the
unit against a
high-precision
10V reference.
to higher precision than its “out of
the box” ±3mV rating. We have made
provision for the trimpot to be added
to the PC board for this purpose but
there is no point in fitting the trimpot
unless you have access to a very high
precision 10V reference.
AD588 availability
That’s about it regarding circuit operation. However, before we move on
to discuss the project’s construction,
a quick word about versions of the
AD588 chip and its availability.
Analog Devices apparently make
five different versions of the AD588,
one in a small outline (SOIC-W) SMD
package and the others in 16-pin ceramic DIL packages. The four CERDIP
devices have different initial error,
Semiconductors
1 AD588AQ or AD588JQ voltage
reference (IC1) – available
from Futurlec (see text)
2 15V 1W zener diode (ZD1,ZD2)
1 5mm green LED (LED1)
2 1N4004 1A diodes (D1,D2)
Capacitors
2 2200μF 25V RB electrolytic
2 470μF 16V RB electrolytic
1 680nF MKT metallised
polyester
2 100nF MKT metallised
polyester
temperature range and temperature
coefficient values. They range from the
AD588BQ with 1mV of initial error, a
-25°C to +85°C range and 1.5ppm/°C
tempco down to the AD588JQ with
3mV of initial error, 0-70°C range and
3ppm/°C tempco. The AD588BQ is
the most expensive (as you would
expect), while the AD588JQ is the
least expensive.
None of the versions of the AD588
seems to be readily available in Australia, especially in one-off quantities.
However, we were able to find one supplier who was able to supply the midrange AD588AQ version (3mV max
initial error, -25°C to +85°C range and
3ppm/°C tempco) for A$28.50 each (at
the time of writing) plus postage. The
supplier concerned is Futurlec, which
Resistors (0.25W 1%)
2 1.8kΩ
1 22Ω
2 68Ω
is based in Broadmeadow NSW but
does all of its business via the web.
So Futurlec is our recommended
source for the AD588AQ. You can
order this part via their website at
www.futurlec.com (item number AD588JN).
Table 1: Resistor Colour Codes
o
o
o
o
siliconchip.com.au
No.
2
2
1
Value
1.8kΩ
68Ω
22Ω
4-Band Code (1%)
brown grey red brown
blue grey black brown
red red black brown
5-Band Code (1%)
brown grey black brown brown
blue grey black gold brown
red red black gold brown
May 2009 65
Below: the PC board is “hung” off the lid of the case on M3 x 25mm tapped
metal spacers and secured using M3 x 6mm screws. Note the “extension”
leads attached to the switch terminals. At right is the view inside the case
with the output terminals mounted and wired back to the board.
If you want to build the unit with
the highest possible precision and
performance, this can be done by using
the BQ or KQ versions of the AD588.
You may be able to order these from
Futurlec but be warned: the BQ version
is considerably more expensive than
the AQ version we have specified and
the KQ version is probably much more
expensive as well.
Construction
Apart from the output terminals, virtually all the components are mounted
on a single PC board measuring 84 x
53.5mm and coded 04305091. This fits
inside a diecast aluminium box (111
x 60 x 54mm), which provides both
shielding and physical protection. The
output and remote sensing terminals
are all mounted on one end of the box,
while the internal/external sensing
switch (S1) is mounted directly on the
lid, with short wire leads connecting
it to the PC board – see photos.
The PC board itself is mounted on
the back of the lid and is supported
via four M3 x 25mm tapped metal
spacers. Unlike switch S1, the power
indicator LED (LED1) mounts directly
on the board and protrudes through a
hole in the lid.
Fig.3 shows the parts layout on the
PC board and the external wiring.
Note that trimpot VR1 is optional, as
mentioned earlier. Note also that IC1
should not be soldered directly into
the board but plugged into a highquality 16-pin DIL socket.
Begin the assembly by installing the
single wire link on the board, then fit
the five fixed resistors, followed by the
Voltage Standards: A Brief History
From 1905 to 1972, the national standard
of EMF or voltage used by the USA was the
Weston Cell, a wet chemical primary cell
or “battery” developed in 1893 by Edward
Weston, of Newark in New Jersey. Weston
had improved on an earlier “voltage standard” cell which had been invented by English
engineer Josiah Latimer Clark in 1873. Weston cells were adopted as the International
Standard for EMF/voltage in 1911.
Weston’s cell had a cadmium-mercury
amalgam anode, a pure mercury cathode,
a paste of mercurous sulphate as the depolariser and a saturated solution of cadmium
sulphate as the electrolyte. It was built in an
H-shaped glass container, with the anode at
the bottom of one “leg” and the cathode in
the other leg. The electrical connections to
the two electrodes were made by platinum
wires fused through the glass at the bottom
of the legs.
The Weston cell provided an accurate
1.0183V reference with a very low temperature coefficient – much lower than Clark’s
earlier cell. However, like the Clark cell, it
66 Silicon Chip
could supply virtually no current and could
only be used to provide a reference voltage for high-resistance measuring circuits
like the classical “potentiometer” (a kind of
bridge which compared a known proportion
of an unknown voltage against the reference
voltage, so no current flowed when the two
voltages were “in balance”).
Weston cells were used as the US and
International standards for EMF/voltage
until 1972, when a new standard came
into use: the Josephson Junction Voltage
Standard (JJVS). This operates on a very
different principle: the phenomenon of
quantum-mechanical tunnelling currents
which flow between two weakly coupled
superconductors separated by a very thin
insulating layer.
This is known as a Josephson junction
and the current is known as the Josephson
current – after British physicist Brian David
Josephson who had predicted the effect in
1962. An improved version of the JJVS was
subsequently developed In the 1980s: the
Josephson Array Voltage Standard or JAVS.
By the way, because Josephson junctions
and arrays depend on superconductivity
for their operation, they must be operated
in a liquid nitrogen environment at 77K
(-196°C).
Essentially, a JAVS forms a frequency-tovoltage converter, whose conversion factor
is exactly reproducible (the agreed figure
is 0.4835979GHz/μV). Because frequency
can be measured extremely accurately
using caesium-beam and caesium fountain
standards, the JAVS therefore provides a
practical voltage standard of similar accuracy. In fact, the estimated accuracy of
current JAVS 10.0V voltage standards is
typically quoted as ±0.017ppm.
More information on the Weston Cell can
be found in Weston’s original US patent
(No. 494,827), available on the US Patent
Office website.
Further information on the Josephson
effect, JJVS and JAVS standards can be
found on http://en.wikipedia.org/wiki/
Josephson_effect and at http://www.
nist.gov/eee1/
siliconchip.com.au
B
B
70
22.75
44.5
C
15
20.5
E
D
CL
11
22.75
B
B
LID, VIEWED FROM ABOVE
three non-polarised MKT capacitors.
The four electrolytic capacitors can
then go in. Be sure to orientate these
as shown in the overlay diagram and
note that the two 2200μF electros are
mounted on their side, with their leads
bent at right-angles to go through their
respective holes in the PC board.
Follow these parts with diodes D1
& D2 and zener diodes ZD1 & ZD2.
LED1 can then be installed. It mounts
vertically with the bottom of its plastic
body about 22mm above the board surface. Be sure to install all these parts
with the correct orientation.
If you are going to use optional trimpot VR1, it can also now be fitted. It
must be installed with its adjustment
screw at lower left (this is to align it
with the adjustment hole drilled in
the lid).
The PC board assembly can now
be completed by installing the DC
power socket (CON1) and the socket
for IC1. Orientate the socket with its
notched end towards the right, as
shown in Fig.3. Leave IC1 out for the
time being.
Preparing the case
Fig.4 shows the drilling details for
the case. Note that the larger holes are
best made by using a small pilot drill to
start with and then carefully reaming
each hole out to its correct size.
Once you have drilled all the holes,
mount the output terminals in place
and tighten their mounting nuts firmly
to prevent them from later coming
loose. That done, solder a short length
(say 75mm) of insulated hookup wire
to the solder spigot at the rear of each
terminal, ready to make the connections to the PC board.
siliconchip.com.au
ALL DIMENSIONS IN MILLIMETRES
CL
HOLES A: 9.0mm DIAMETER
HOLES B: 3.0mm DIAMETER, CSK
HOLE C: 5.0mm DIAMETER
HOLE D: 3.0mm DIAMETER
HOLE E: 6.0mm DIAMETER
CL
A
A
16.25
10
A
19
A
A
9.5
LEFT-HAND END OF BOX
9.5
RIGHT-HAND END OF BOX
Fig.4: the drilling details for the case. Use a pilot drill to start the larger holes
then step the up to the correct size using a larger drill and a tapered reamer.
Next, attach the front-panel label
to the lid. This label can be made by
downloading the artwork from the
SILICON CHIP website, printing it out
and then covering it with a protective
self-adhesive transparent film. Attach
the label using a thin smear of silicone
sealant, then cut out the holes using a
sharp hobby knife.
Toggle switch S1 can now be mounted in position on the lid. Tighten its
mounting nut firmly, then fit six 15mm
lengths of tinned copper wire to its
connection lugs (these leads later pass
through their corresponding holes in
the PC board). Loop the end of each
wire through the hole in its switch lug
before soldering, to make sure these
joints can’t come adrift when the outer
ends of the wires are soldered to the
board pads.
The next step is to attach an M3 x
25mm tapped metal spacer to each
corner of the PC board. Secure these
using four M3 x 6mm pan-head machine screws, then install the leads
between the output terminals and
their corresponding PC board pads –
see Fig.3.
IC1 can now be plugged into its
socket. Be sure to orientate it correctly
and make sure that all its pins go into
the socket – ie, not down the outside
of the socket or folded under the IC
itself. The PC board can then be attached to the lid.
Note that the extension wires fitted
to switch S1 must all pass through
their matching holes in the PC board,
while LED1 must pass through its
corresponding hole (C) in the lid.
Secure the board to the lid using four
countersink-head M3 x 6mm screws,
then solder the six switch leads to their
board pads.
The unit can now be completed by
May 2009 67
STIC
FANTAIDEA
GIFT UDENTS
FOR SFT ALL
O S!
AGE
THEAMATEUR SCIENTIST
An incredible CD with over 1000 classic projects
from the pages of Scientific American,
covering every field of science...
NEW VERSION 4 –
JUST RELEASED!
GET THE LATEST
VERSION NOW!
Arguably THE most IMPORTANT collection
of scientific projects ever put together!
This is version 4, Super Science Fair Edition
from the pages of Scientific American.
As well as specific project material, the CDs
contain hints and tips by experienced amateur
scientists, details on building
science apparatus, a large
database of chemicals and
so much more.
ONLY
62
$
00
PLUS $10 Pack and Post
within Australia
NZ P&P: $AU12.00,
Elsewhere: $AU18.00
“A must for every science student,
science teacher, science lab . . . or simply
for those with an enquiring mind . . .”
Just a tiny selection of the incredible range of projects:
! Build a seismograph to study earthquakes ! Make soap bubbles that last for
months ! Monitor the health of local streams ! Preserve biological specimens !
Build a carbon dioxide laser ! Grow bacteria cultures safely at home ! Build a
ripple tank to study wave phenomena ! Discover how plants grow in low gravity !
Do strange experiments with sound ! Use a hot wire to study the crystal structure
of steel ! Extract and purify DNA in your kitchen !Create a laser hologram ! Study
variable stars like a pro ! Investigate vortexes in water ! Cultivate slime moulds !
Study the flight efficiency of soaring birds ! How to make an Electret ! Construct
fluid lenses ! Raise butterflies as experimental animals ! Study the physics of
spinning tops ! Build an apparatus for studying chaotic systems ! Detect metals in
air, liquids, or solids ! Photograph an ant's brain and nervous system ! Use
magnets to make fluids into solids ! Measure the metabolism of an insect . . . !
and many, many more (a thousand more, in fact!)
See the V2 review in SILICON CHIP, October 2004. . . or read on line at siliconchip.com.au
This is the ALL-NEW Version 4 . . . it’s even BETTER!
HERE’S HOW TO ORDER YOUR COPY:
BY PHONE:*
BY FAX:#
(02) 9939 3295
9-5 Mon-Fri
<at>
(02) 9939 2648
24 Hours 7 Days
BY EMAIL:#
silicon<at>siliconchip.com.au
24 Hours 7 Days
BY MAIL:#
BY PAYPAL:#
PO Box 139,
Collaroy NSW 2097
silicon<at>siliconchip.com.au
24 Hours 7 Days
* Please have your credit card handy! # Don’t forget to include your name, address, phone no and credit card details.
BY INTERNET:^
siliconchip.com.au
24 Hours 7 Days
^ You will be prompted for required information
There’s also a handy order form inside this issue.
Exclusive in SILICON
Australia to: CHIP siliconchip.com.au
68 Silicon Chip
siliconchip.com.au
fastening the lid/PC board assembly to
the box using the screws supplied.
Internal & External Sensing Connections
Using it
There are no adjustments to be made
to the Precision Voltage Reference if
you don’t have access to a very high
precision voltage source to calibrate it
against. As stated previously, without
calibration, it will operate with better
than ±3mV precision, as provided by
the AD588AQ chip itself.
In that case, it’s merely a matter of
switching S1 to the internal sensing
position and applying power (12V
AC) to CON1. LED1 should light immediately to show that the unit is
operating and 10.000V ±3mV will
now be available at the upper output
terminals, ready for checking your
DMM or whatever.
This “internal sensing” configuration is the one to use for most simple
jobs like DMM calibration, with the
DMM input leads connecting directly
to the Precision Voltage Reference’s
upper output terminals.
Cable compensation
The only occasions when it’s preferable to use external sensing or “Kelvin
connections” will be when you are
supplying the unit’s voltage to a load
at the end of a cable and the load is
drawing sufficient current to introduce
a significant voltage drop due to the
cable resistance.
In such situations, you’ll need to
extend the output sensing terminals
of the Precision Voltage Reference to
+OUT
10.00
+SENSE
INT
DC VOLTS
EXT
SENSING
DMM
–OUT
–
–SENSE
+
PRECISION VOLTAGE REFERENCE
A LOCAL MEASUREMENT, INTERNAL SENSING
DMM
LONG CABLES
10.00
DC VOLTS
+OUT
+SENSE
INT
EXT
SENSING
–
LOAD
–SENSE
PRECISION VOLTAGE REFERENCE
B REMOTE MEASUREMENT, EXTERNAL 'KELVIN' SENSING
Fig.5: how to connect the Precision DC Voltage Reference for both local (A)
and remote (B) measurements (the latter compensates for cable losses).
the load end of the cable via a second
pair of leads as shown in Fig.5. Then
S1 is switched to the external sensing
position, so that the AD588 senses the
output voltage right at the load end of
the cable rather than at its own end.
As a result it will maintain the load
voltage at the correct 10.000V, compensating for the cable drop.
All of the foregoing also applies if
you build the unit with trimpot VR1
and/or use an AD588BQ/KQ for higher
precision. The only complication in
these latter situations is that you’ll
need to compare the output of the
Precision DC Voltage Reference with
a higher precision source and adjust
VR1 to trim its output as close as possible to 10.0000V before you can put
SC
it to use.
Australia’s Best Value Scopes!
Shop
On-Line
at
emona.com.au
GW GDS-1022 25MHz
RIGOL DS-1052E 50MHz
RIGOL DS-1102E 100MHz
25MHz Bandwidth, 2 Ch
250MS/s Real Time Sampling
USB Device & SD Card Slot
50MHz Bandwidth, 2 Ch
1GS/s Real Time Sampling
USB Device, USB Host & PictBridge
100MHz Bandwidth, 2 Ch
1GS/s Real Time Sampling
USB Device, USB Host & PictBridge
Sydney
Brisbane
Perth
ONLY $599 inc GST
Melbourne
Tel 02 9519 3933
Tel 03 9889 0427
Fax 02 9550 1378
Fax 03 9889 0715
email testinst<at>emona.com.au
siliconchip.com.au
+
–OUT
ONLY $879 inc GST
Tel 07 3275 2183
Fax 07 3275 2196
Adelaide
Tel 08 8363 5733
Fax 08 8363 5799
ONLY $1,169 inc GST
Tel 08 9361 4200
Fax 08 9361 4300
web www.emona.com.au
EMONA
May 2009 69
|