Tag: FR

Differential buffer and preamp

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:

SSM2019 preamp
SSM2019 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

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Sweep buffer

I’ve been using my sound card and Pete Millett’s interface for testing. However, the limitation on high frequency response is due to the sound card. A cheap, but yet effective option, is to use the Chinese digital oscilloscope Instrustar ISDS2062B which comes with built-in DDS signal generator with sweep capability. With this little piece of technology, you can sweep up to 10Mhz. The resolution is 12bit, not great for FFT but good enough for a FR analysis. For frequencies above 20kHz, you can use this device to look into things. For FFT and THD analysis, I will keep using the sound card and audio interface as usual.

When I test the device, I found that the DDS output didn’t have the stones to drive loads at HF. Therefore I went back to my workshop and built the following sweep driver. This was based on the great SSM2019 that my friend Mogens sent me:

Sweep buffer based on SSM2019
Sweep buffer based on SSM2019

The circuit is very simple. It’s operated from a pair of 9V batteries. So far, I’ve tested with the 20dB (actually 19dB gain due to 1kΩ resistor at hand) and 0dB gain modes.

The response is good enough for my purposes:

SSM2019 sweep test

I can get 1.5Mhz @ 19dB or 3Mhz @ 0dB gain HF response which is great. I now need to test it again with a real load.