|
Edited by gf1 at 2019-4-10 07:02
I'd like to come back to this low-frequency scope noise issue when the AWG is driving a load.
When I increase the AWG frequency to about 5500 Hz, then I get this picture:
Again there is no signal connected to both scope inputs, but the scope inputs are 50 Ohm terminated (in order that the BNC connectors don't pick up EMI). The AWG is set to 5500 Hz square wave, 2.3V amplitude, and drives a 50 Ohm load. The peak-peak noise amplitude on both, CH1 and CH2 is almost 1 div , when the channels are set to either 10mv/div or 500mv/div.
Here's what I've analyzed so far:
- If the AWG is driving a load (particularly with square wave with fast risetime), is is of course unavoidable that it creates load transients on the power supply rails.
- The DC converter for the negative supply has obviously a rather small phase margin, so that its load transient response shows significant overshoot/ringing, with crossover frequency of about 5500 Hz. When the AWG is set to 500 Hz, the overshoot is about 40mV pp. With an AWG frequency of ~5500 Hz, we hit the resonance frequency of the DC conveter, and amplitude is even higher (-> above image).
- Besides the dynamic transient response, there is also a small residual ohmic voltage drop on the power supply rail which follows the AWG waveform. With the AWG set to 500 Hz, this component is only about 7mV pp (but even this is sufficient to couple about 2mV pp into the scope frontend).
- The power supply rails of the AWG and the scope frontends are not sufficiently decoupled from each other.
- The FET buffer amplifier in the frontends has a very low PSRR with regard to V- at low frequencies (not more than ~10dB). The weak spot are the two current sinks which are biased by a resistive devider between the (noisy) V- and GND, copuling the noise into the constant current. There is a bypass capacitor, but it is much to small to help at low frequencies. There is also the DC compensation path for the FET buffer via the opamp, but it joins in at only even lower frequencices (< 20 Hz ?) and does not help at say 500 Hz.
So far I could achieve the best mitigation by adding an additional 100uF bypass from the bias point of the current sinks in the frontends to V-, and adding 1000uF low ESR from V- to GND near the AWG's output amp (where the load transients originate). Well, 1000uF is of course pretty large. Alternatively I tried to add a feedforward capacitor of 10nF to the V- DC converter. This does indeed improve the ringing and the amplitude of transient response, reducing the amplitude of the 5500 Hz scope noise (image above) from 1 div to 1/3-1/2 div pp, but this is still too much, and the 1000uF bypass still performes better. I'm still investigating/trying...
[ Btw, I'm talking about low-frequency noise. High-frequency noise ist yet another issue. ]
|
This post contains more resources
You have to Login for download or view attachment(s). No Account? Register
x
|