Tektronix ADA400A Differential Preamplifier repairs

Differential Preamplifier

The ADA400A Differential Preamplifier improves oscilloscope capability to measure low amplitude signals which may not be ground referenced. The high impedance of both inputs eliminates the need to add additional ground points in the DUT, thereby avoiding ground loops which disturb sensitive circuits.

Features and specifications:

  • Gain settings: 100x, 10x, 1x, 0.1x
  • Max common mode signal input ±40V on 1x and 0.1x gain or ±10V on 100x and 10x gain
  • Differential signal range 0.1V (A=100x) / 1V (A=10x) / 10V (A=1x) / 80V (A=0.1x)
  • Max offset ±40V on 1x and 0.1x gain or ±1V on 100x and 10x gain
  • Selectable bandwidth limits : 100 Hz, 3 kHz, 100 kHz, >1 MHz
  • Noise : Typically less than 30 µVRMS at gain 100x (referred to input)
  • Input impedance : 1 MΩ || ~55 pF (Hi-Z infinity Ω selectable in 100x and 10x gains)
  • Best sensitivity 10 µV/division (100x gain)
  • Typical 100 dB CMRR (DC to 10 kHz)
  • Active differential preamplifier
  • Internal probe power with TEKPROBEBNC ports
  • Compatible with Tektronix TDS400, TDS500, TDS600, TDS700, TDS5000, TDS7000 Series
  • Can be powered by Tektronix Model 1103 PSU

MSRP of this probe from Tektronix online store is $2310 USD.

There is clear indication of previous intruders. Poor Linear LT1021-5CN is butchered with lot of ugly flux residue on pins.

Measurement of the output confirmed this reference very dead. At +15V input it provided only 0.675V output. Supposed to be 5V!

Also by accident I overloaded the input with 82 VAC, and tripped GDT protection. Well, at least we know it works!

This preamplifier have limited compatibility with newer Tektronix TekVPI oscilloscopes, such as our DPO7104C. Adapter TPA-BNC must be used, and gain selection must be done ONLY before plugging probe into scope. Switching gain while connected likely to damage TPA-BNC adapter’s +5V regulator due to excessive current spike!

Installed new reference, confirmed nice and stable +5.000 V output

Now ADA400A is fully functional, and we can measure 2 mV 100 kHz sinewave from Fluke 5720A calibrator output.

Direct measurement by scope, at most sensitive 1 mV/div range:

And same signal measured with Tektronix ADA400A as preamplifier.


Other links: https://github.com/mattico/ADA400A-Reversing

Author: Ilya Tsemenko
Created: April 28, 2020, 2:02 a.m.
Modified: July 12, 2021, 2:17 p.m.