Page 110 - Troubleshooting Analog Circuits

Measure Bias Current Rather Than Impedance 97

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I’ve measured some input capacitances and find the circuits of Figure 8.8 to be
quite useful. Input-capacitance data is nominally of interest only for high-impedance
high-speed buffers or for filters where you want to make sure that the second-source
device has the same capacitance as the op amps that are already working okay.

Recognize False “Error” Characteristics

Sometimes, an op amp may exhibit an “error” that looks like a bad problem. but isn’t.
For example, if you have your op amp’s output ramping at 4.3 V/pec, you might be
surprised when you discover that the inverting input, a summing point. is not at
ground. Instead, it may be 15 or 30 or 100 mV away from ground. How can the offset
voltage be so bad if the spec is only 2 or 4 mV?
Why isn’t the inverting input at the “virtual ground” that the books teach us? The
virtual ground theory is applicable at DC and low frequencies, but if the output is
moving at a moderate or fast speed, then expecting the summing point to be exactly
at ground is unreasonable. In this example, dV,,,/dt equals 2x times the unity-gain
frequency times the input voltage. So, 15 mV of Vi,, is quite reasonable for a
medium-bandwidth op amp, such as an LF356, and 50 or 70 mV is quite reasonable
for an LM741. If you want an op amp to move its output at any significant speed.
there has to be a significant error voltage across the inputs for at least a short time.