Signal conditioning Outputs Figure 2. A classic 3-op-amp in-amp. Comparators usually do not have a negative

feedback network, so the simple R and C in front of the comparator in Figure 5 forming a low-pass filter works well. RHYS should be much larger than R7, and the two divide the output swing to provide a small amount of positive feedback (hysteresis). If the comparator has built-in hysteresis, such as the LTC6752 or the ADCMP391, then R7 and RHYS are not used.

Figure 3. Classic feedback schematic.

Open-LOOp and CLOsed- LOOp Gain

For op amps, looking at Equation 1, the higher

the open-loop gain (AVOL), the more accurate the closed-loop gain will be. Most op amps have open-loop gain between 100,000 and 10 million, but some of the older high speed op amps might be as low as 3,000. As shown earlier, the higher the open-loop gain, the lower the closed-loop gain error. For a comparator, if the logic swing on the

output is 3 V, and you want a 1 mV threshold, then the minimum gain needs to be 3,000. Higher gain will give you a smaller window of uncertainty, but if the gain is too high, microvolts of noise will trigger the comparator. For an instrumentation amplifier, the concept of open-loop gain does not really apply.

An op amp or an instrumentation amplifier will have an output that swings from close to one rail and to the other. Depending on whether the output stage uses common emitter or common source configurations, it may get within 25 mV to 200 mV of either rail. This would be considered a rail-to-rail output. If the op amp is powered by +15 V and –15 V, this is inconvenient to interface to digital circuitry. One poor solution that has been tried is to put diode clamps on the output to protect the digital input from damage. Instead, the op amp current goes sky high and the op amp gets damaged. There are more elaborate ways to interface an op amp to digital logic, but why bother? Just use a comparator. Comparators can have a CMOS totem-pole

output, or an NPN or NMOS open-collector or open-drain output. Although the open-collector or open-drain output requires a pull-up resistor, resulting in unequal rise and fall times, it does offer the advantage of operating the comparator on one voltage, say 5 V, and interfacing to logic operating on a different voltage, such as 3.3 V.

impOrtant speCs

Figure 5. Comparator with LPF and hysteresis. For instrumentation amplifiers, a cap across

the inputs is quite acceptable as shown by C4 in Figure 6. The figure in Chapter 5 of the Analog Devices instrumentation guide shows a good thing to do every time you use an instrumentation amplifier. If you lay out the printed circuit board with the appropriate traces and pads to allow adding the two resistors and three capacitors, you can start with 0 Ω resistors and no capacitors, and measure the system performance. By adjusting the values of the five components, you can set the common-mode roll-off and the normal-mode roll-off independently (see the guide for details).

For an op amp, we need a gain bandwidth higher than the highest signal frequency to keep the closed-loop error low. Looking at Equation 1, we can see where the rule of gain bandwidth should be 10 to 100 times the highest signal frequency. From Equation 1, as discussed earlier, note that AVOL is a function of frequency and will affect the closed-loop accuracy. Phase margin is also important and will vary with capacitive load, so the spec table should clearly state the test conditions. For dc accuracy, the offset voltage should be low. For a trimmed bipolar op amp, 25 μV to 100 μV is good; for a FET input op amp, 200 μV to 500 μV is good. Auto-zero/chopper/zero-drift op amps are almost always below 20 μV maximum, and this is over temperature. For examples, see some typical op amp data sheets, such as the OP27, AD8610, or ADA4522.

Continued on page 56...

Figure 4. An attempt to reduce op amp bandwidth. input CapaCitOrs

Capacitors are often added to circuits to limit the bandwidth. Looking at Figure 4, at first glance it seems that R1 and C1 form a low-pass filter. This does not work and can lead to oscillations. The feedback factor for an inverting amplifier is R2/R1, but in Figure 4, the feedback factor is R2/(R1 + Xc). As the frequency increases, the feedback factor increases, so the noise gain is going up at +20 dB/decade, while the op amp open-loop gain is going down at –20 dB/decade. They cross at 40 dB, which, according to control system theory, guarantees oscillation. The correct way to restrict the bandwidth of the circuit is to put the capacitor across R2.

Instrumentation Monthly March 2021

Figure 6. RFI filter before instrumentation amplifier.


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