The ADA400A has a TekProbe interface. It can either be used directly in an oscilloscope that has this interface or through a power supply like the Tektronix 1103. Ever since I got my ADA400A I wanted to build a proper power supply for it in order to use it with my oscilloscope or dynamic signal analyzer. Ideally I want it be powered from a USB port because in most modern instruments there is a port available. For the time being I just installed a cable inside the amplifier and power it from there. You can find the pinout in the next picture:
![](https://btbm.ch/content/images/2021/01/cable.jpg)
The amplifier uses the +15V, -15V and +5V lines. The +5V are used in the 0.1 and 1 gain settings. If you plan to use the ADA400A only with the gains of 10 and 100 you can power it with just +15V and - 15V. I allege the 5V are used to switch the relays in the front end.
![](https://btbm.ch/content/images/2021/01/ADA400A-2.jpg)
You can see the current consumption for each rail in the picture below. All together the power consumption is about 0.8W. A normal USB port can provide about 2.5W so even if we assume a moderate efficiency of 50% there is plenty of power.
![](https://btbm.ch/content/images/2021/01/ADA400A-Power-requirements.jpg)
Why do I need a differential preamplifier?
Asked the wife... If you don't know take a look at the picture below.
![](https://btbm.ch/content/images/2021/01/ScreenImg-1.png)
Both oscilloscope channels probe the same 2mVp-p signal. The yellow one is probed with the ADA400A. The pink goes to the oscilloscope directly (50Ohm terminated). The difference in noise is obvious.
Another need that a differential preamp serves is to probe signals that are not referenced to ground.
Bode plots for the ADA400A
Since I was playing around with the probe I thought I would make some bode plots with my SIGLENT SDS2104X+.
![](https://btbm.ch/content/images/2021/01/ADA400-bode-full-G0.1.png)
![](https://btbm.ch/content/images/2021/01/ADA400-bode-full-G1.png)
![](https://btbm.ch/content/images/2021/01/ADA400-bode-full-G10.png)
![](https://btbm.ch/content/images/2021/01/ADA400-bode-full-G100.png)