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Impedance measurements stabilize op-amp buffers

Figure 1. Op amps are often used as unity gain signal buffers, which are not as simple to use as it may appear.

The unity-gain op amp (operational amplifier) buffer circuit is routinely used to transform high-impedance inputs into low-impedance outputs. ADC (analog-to-digital converter) voltage references are also frequently buffered to reduce the voltage reference noise resulting from the ADC sampling current.

When implemented well, using an appropriate op amp, the results work out nicely. More frequently than not, however, the op amp (Figure 1) suffers from degraded stability caused by output capacitive loading either from components or frequently from the capacitance of a multilayer PCB. The degraded stability results in high output impedance and high output noise, the exact opposite of the circuit’s intended function. To improve stability, start by measuring the op amp’s impedance.

The measurement Measuring the stability of this circuit can be challenging because of the circuit’s wide bandwidth and the interaction between the op-amp circuit and the PCB. As a result of the interaction, we would ideally like to measure the impedance of the unity-gain buffer while in the circuit. Otherwise, we would need to precisely replicate the circuit board loading in a test fixture. Measuring in-circuit is faster and requires much less effort than replicating actual conditions in a test fixture.

Bandwidth is another challenge. In this example, the bandwidth is nearly 200 MHz, well above the range of an FRA (frequency-response analyzer) such as those from Ridley Engineering or Venable Instruments. Even if the circuit could be measured using an FRA, the interconnections between the instruments and the PCB would be complicated at 200 MHz.

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