Another technique for improving IMD involves modifying the auto-zero clock. By using a randomized clock frequency, as opposed to a fixed clock frequency, the clock feedthrough and IMD energy is randomized as well. If the auto-zero clock is optimized with a uniform probability density function, then IMD disappears. The noise floor of the amplifier will increase slightly, however. Conceptually, the energy in the IMD and clock feedthrough spurs is redistributed, smeared across all frequencies and showing up as broadband noise. Figure 8 shows the spectral output of the AD8572, which uses such a technique to eliminate IMD.
Intermodulation distortion and clock feedthrough are not the only side effects to using auto-zero correction. The charge injection from the auto-zero clock can create current spikes on the input bias current. The current by itself is not a major issue, but it does create a voltage spike at the input that is proportional to the size of the resistors used around the amplifier. For this reason, excessively large resistors are not recommended as it could cause excessive offset voltage spikes.
Applications for Auto-Zero Amplifiers
We've talked a bit on auto-zero technology, now we'll shift our focus on applications for these devices. The most obvious is any kind of precision or high gain application. Weigh scales have already been mentioned, but also strain gauges, pressure measurement, and other instrumentation apps.
Other applications that are often overlooked are ones that may not necessarily require absolute precision, but cannot tolerate any drifts over temperature. This is found in many automotive applications where there may be wide swings in operating temperature. The continuous correction of auto-zero amplifiers makes them a natural fit for such applications with the added benefit of having high precision as well. This same principle hold true for zero drift over lifetime circuits, such as remote sensors, where routine calibration is not practical.
Some applications require good ac performance, but also need low dc offset voltage. A precision current sensor is an example of one such application, as shown in Figure 9. Auto-zero amplifiers lend themselves well to such apps, but the intermodulation distortion of the op amp must be considered. Non-fixed correction frequency auto-zero op amps are the best choice here to minimize IMD. Again, the penalty for lowering IMD is an increase in the noise floor, lowering the overall resolution slightly.
Despite their internal complexity, auto-zero amplifiers can be configured and
