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“Over-The-Top” rail-to-rail op amps by Nicholas Vinen In our Deluxe Touchscreen eFuse project this month, we’re using two “Over-The-Top” rail-to-rail op amps which provide functions available in few other op amps. Made by Linear Technology (now part of Analog Devices), they are very useful in instrumentation applications. O p amps are one of the most common types of IC. We est-imate that there are close to 10,000 different types available; if you discard those which are related (single/dual/ quad versions, for example) there are still more than 1000 distinct designs. So it’s unusual to have design criteria so strict that you are only left with one or two suitable types. The combination of attributes which make these “Over-The-Top” op amps useful in the eFuse would also make them valuable in other instrumentation roles. The particular op amps have the following type codes: LT1490A/LT1491A and LT1638/LT1639, representing dual and quad versions respectively. The major difference between the two pairs is the trade-off between bandwidth, noise and power consumption. One of their unusual features is the fact that both the differential and common mode input range is 44V, regardless of the op amp’s supply voltage. So you could use these op amps to measure the voltage across a shunt that is supplying the high side of a motor running off 36V DC, even if your op amps are only running off a 3V supply. That’s why they’re called “Over-The- they do have much more flexibility than a difference amplifier, providing traditional op amp functions, along with a much higher input impedance for more accurate measurements. Other notable features Top” and it’s a feature normally reserved for what is called a “difference amplifier” (as distinct from an operational amplifier). Difference amplifiers are similar to instrumentation amplifiers but they lack input buffering, having an internal precision divider between each input and an internal instrumental amplifier. One example is the INA117 from Texas Instruments. So difference amplifiers are capable of handling very high input voltages and tend to have very good common mode rejection ratios (CMRR), in order to allow them to accurately measure small differences between those input voltages. However, they are quite restricted in their applications, as they often have fixed gain and the higher the allowable input voltage, the higher the gain tends to be. While the LT1490/1490/1638/1639 can’t handle particularly high voltages, While the Over-The-Top feature is interesting, that isn’t actually why we chose these devices for the eFuse. The main reason is their combination of a very wide operating supply voltage range, from 2V to 44V (!) along with railto-rail inputs and outputs, with an output which can swing close to each rail (maximum 10mV) and a low typical input offset voltage of ±110µV (maximum ±800µV from -40°C to +85°C). Two of the biggest drawbacks of traditional rail-to-rail op amps are their limited supply voltage range (usually 2.7-16V; the LMC6482 we often use has a rating of 3-15.5V) and the fact that the output voltage will only swing close to either supply rail, but not actually reach it. While it’s impossible for an op amp output to actually reach either of its supply rails, the op amps described here can get very close, typically to within about 3mV of the negative rail when lightly loaded, as you can see from Specifications (typical figures) LT1490A/LT1491A LT1638/LT1639 Supply voltage range (Vs) ........................ Quiescent current ..................................... Gain bandwidth product ........................... Slew rate ................................................... Input offset voltage.................................... Input bias current ..................................... Input noise voltage ................................... Large signal voltage gain........................... Output swing, no load............................... Output swing <at> 5mA................................. Output short circuit current....................... PSRR ........................................................ CMRR <at> 1kHz .......................................... 2.4-44V 40µA/amplifier 200kHz 60mV/µs 150µV (250µV for LT1491A) 1nA 50nV/√(Hz) 1500V/mV <at> Vs=3-5V, 250V/mV <at> Vs=30V 3mV to Vs-12mV 250mV to Vs-600mV +15, -30mA (Vs=3V), +25, -30mA (Vs=5V) 98dB 92dB 2.4-44V 170µA/amplifier 1.2MHz 380mV/µs 250µV (350µV for LT1639) 20nA 20nV/√(Hz) 1500V/mV <at> Vs=3-5V, 500V/mV <at> Vs=30V 3mV to Vs-20mV 250mV to Vs-600mV +15, -25mA (Vs=3V), +20, -25mV (Vs=5V) 100dB 103dB 60  Silicon Chip siliconchip.com.au Fig.1. By comparison, the LMC6482’s output saturation voltage is similar when sinking 100µA+ but only drops down to around 10mA when sinking just 1µA. While the LM358 isn’t a rail-to-rail op amp, it is designed for operation from single supplies and was one of the earliest designs to have an output swing that came close to the negative rail. It’s still in common use but it too struggles to deliver an output voltage below 10mV. CHARGER VOLTAGE RS 0.2Ω RA 2k IBATT RA´ 2k Q1 2N3904 + 1/4 LT1491A – – 1/4 LT1491A LOGIC + RB 2k Q2 2N3904 + RB´ 2k LOGIC HIGH (5V) = CHARGING LOGIC LOW (0V) = DISCHARGING 1/4 LT1491A – LOAD + + RG 10k VBATT = 12V S1 10k VOUT 1/4 LT1491A – 90.9k 1490A TA01 Power supply, bandwidth and noise VOUT V IBATT = = OUT AMPS (RS)(RG/RA)(GAIN) GAIN S1 = OPEN, GAIN = 1 RA = RB S1 = CLOSED, GAIN = 10 VS = 5V, 0V Fig.2: an example circuit from the LT1490 data sheet which takes advantage of the “over-the-top” capability of these op amps. The LT1490/1491 have a low power consumption figure of just 40µA/ amplifier and 170µA/amplifier for the LT1638/1639. The trade-off in achieving this is in the bandwidth and noise figures. The LT1490/1491 have a gain bandwidth (GBW) product of just 200kHz while the LT1638/1639 have a GBW of 1.075MHz. Noise figures are 50nV/√(Hz) for the LT1490/1491 and 20nV/√(Hz) for the LT1638/1639. But for instrumentation purposes like our eFuse, those figures are more than adequate. A bandwidth of say 50kHz (ie, with an effective gain of four) still results in a 0.1% settling time of around 20µs. So if you are feeding the op amp output to an analog-todigital converter (ADC) in a microcontroller, unless you’re sampling above 50kHz, it could be an advantage as it will act as a low-pass filter to reduce aliasing in the ADC. Another unusual feature of the op amps described here is that they will tolerate a reverse supply condition (ie, V+ below V-) with less than 1nA of current flow for reverse voltages up to 18V. So they could be used in batterypowered applications and powered directly off the battery without concern for damage if it were to be accidentally reversed. No damage will occur with input voltages down to -2V. And they will tolerate up to 18V on all input and output pins in the absence of supply voltage, allowing them to be “shut down” by switching V+ off using a transistor. They will also tolerate driving a capacitive load of up to 200pF, with no extra compensation, or up to 10nF (LT1490/1) or 1nF (LT1638/9) with an added Zobel network at the output. High open-loop gain (1.5 million times) and CMRR (98dB), along with phase reversal protection, makes these op amps suitable for precision DC work. They also have a reasonably strong out- Output Low Saturation Voltage vs Load Current 1000 put drive, of ±25mA, rising to ±40mA at higher supply voltages. For AC/audio applications, total harmonic distortion (THD+N) is quite low at around 0.002%, limited mainly by noise. Conclusion These op amps are excellent general purpose devices and come about as close to an “ideal op amp” as we’ve seen. They have little change in performance over a wide range of supply voltages and their high maximum supply voltage makes them very useful in circuits with multiple supply rails. It also means that they will be useful in a variety of situations, whether you’re building a circuit which runs off a single Li-ion cell, a 12V power supply or with substantially higher voltage rails. We expect we will use this family of op amps in more projects in future. They are available from Digi-Key (DK) and element14 (e14), with catalog codes as follows: LT1490ACN (dual, 200kHz, DIP) – DK LT1490ACN8#PBF-ND; e14 9560530 LT1490ACS (dual, 200kHz, SOIC) – Fig.1: a plot of the typical output voltage of four different op amps when fully to the negative rail versus load current. The LT1490/1638 op amps go lower than most other high-voltage-capable railto-rail op amps. Note that to take advantage of this, the op amp output must be very lightly loaded. siliconchip.com.au Output Saturation Voltage (mV) TA=25°C DK LT1490ACS8#PBF-ND; e14 1663433 VS=5V LT1491ACN (quad, 200kHz, DIP) – 100 DK LT1491ACN#PBF-ND; e14 9560556 LT1491ACS (quad, 200kHz, SOIC) – DK LT1491ACS#PBF-ND; e14 1330667 LT1638CN (dual, 1.2MHz, DIP) – LT1490 10 DK LT1638CN8#PBF-ND LT1638 LT1638CS (dual, 1.2MHz, SOIC) – LMC6482 DK LT1638CS8#PBF-ND; e14 1663461 LM358 LT1639CN (quad, 1.2MHz, DIP) – DK LT1639CN#PBF-ND 1 0.1µ 1µ 10µ 100µ 1m Sinking Load Current (A) 10m 100m LT1639CS (quad, 1.2MHz, SOIC) – DK LT1639CS8#PBF-ND; e14 1330682 SC July 2017  61