Question
We are using the ADM7171 in our receiver design. The current layout uses a 10uFceramic cap on the output, but due to microphonic problems (the regulator feeds asensitive VCO), I changed to a 4u7 tantalum cap. Since the ESR is high (5R), Iadded a 100nF in parallel to lower the effective ESR. However, the circuitoscillates at ~7 MHz. The ceramic's ESR is below the recommended 50 mR from1-72 MHz. What is the required frequency range or how can I determine asolution that will give an acceptable production yield? Regulation is from 6Vdown to 5V, with 15kR and 4k7R regulator divider resistors.
Answer
Under your configuration, I would recommend at least a 1uF ceramic (Low ESR) capacitor in parallel with the Tantalum to maintain adequate stability.I was able to model in simulations the frequency response, and it definitely goes unstable with a very high ESR tantalum capacitor.
Now, Murata has low microphonic ceramic capacitors that you may want to check out, with lower ESR than tantalums (and smaller size). We’ve discussedthese caps with Murata many times in the past, and even evaluated LDO performance/stability/etc, but did not study/evaluate the microphonicperformance. Although, I do remember Murata promoting a reduction of acoustic noise by ~20dB (ZRA series) compared with standard ceramic capacitors.Murata calls these “Acoustic Noise Reduction Capacitors”, and they can be found under the following series of capacitor names:
GJ8, KRM, ZRA