January 17, 2021 Power Supplies, Test Equipment

Powering the Tektronix ADA400A Differential Preamplifier

Adding a cable to the ADA400A to connect it to an external power supply

Powering the Tektronix ADA400A Differential Preamplifier

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:

ADA400A power supply pinout
The TekProbe interface

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.

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.

ADA400A power requirements

Why do I need a differential preamplifier?

Asked the wife... If you don't know take a look at the picture below.

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+.

G = 0.1
G = 1
G = 10
G = 100