As part of improving my bench test gear to do sweep tests and impedance measurements, I ended up building a great preamp and buffer gig based on the SSM2019 device as described previously here.

Here is the main circuit for the preamp:

 

The circuit is same as described before. I added a rotary switch to select gain from 0dB to 60dB (ignore the diagram labels). The circuit has a DC input (differential floating) and an AC input for voltages less than 60V. There is a switch for AC/DC selection and also a switch to ground the negative input for DC mode when we don’t want it floating.

The preamp AC output is not shown but is a 1uF with a 220k resistor to ground.

The buffer circuit is similar to the above but without AC circuit and no gain selection.

Build

I used a small aluminium box to fit the whole thing. I drilled both sides to fit the BNC connectors, switched etc. A bit tight but all fit:

The circuit was built on 3 breadboard PCBs: one for the buffer, one for the preamp and one for the AC coupling circuit:

 

 Testing

The differential preamp jig works great. Here are some tests:

Starting with the sound card loopback as a reference, the 1kHz distortion is as low as 0.0021%:

When we look at the buffer output:

H2 distortion is increased by 8dB. Rest of harmonics remains the same roughly. Overall THD has risen now to 0.0033%.

If we use the AC coupling input distortion is increased slightly;

2dB for H2 and step increase for H3. Overall THD up to 0.0043% or 0.001% increase.

Now, let’s look at the buffer feeding the preamp configured at 0dB gain:

Overall THD is still very good and just increased to 0.0046%.

Noise floor tests

If we short the input we can look at the noise floor of the preamp in the various gain modes:

This is beautiful. Noise floor is down to less tan 640nV for 60dB gain. The workbench noise is filtering through as well as the MiniPC USB power supply ripple through the USB. With a well filtered and floating DC supply for the Sound card interface this can get even better!

Now if we couple the input in AC mode, noise floor can be as low as 140nV:

 

I think I have now what I needed to improve the bench measurements!