Tektronix ADA 400A Differential Preamplifier

[EC8010]

The ADA 400A was mentioned at the other place and was the subject of some haphazard reverse engineering. Essentially, it's a differential input (two BNCs) to single-ended output on TekProbe interface. Gain is variable; 0.1, 1, 10, 100, and bandwidth is variable; 100Hz, 3kHz, 100kHz, >1MHz, but the important feature is the 100dB CMRR at low frequencies. I have a 'scope with TekProbe interface, so I ordered one (second-hand) and it arrived this afternoon. Oh how I hate calibration houses and their tamper-proof stickers! It is not necessary to put one over each of the four screws holding the lid on; one will do. It took half an hour, four lollypop sticks, copious kitchen roll, Sticky Stuff Remover, and isopropyl alcohol to get those stickers and associated goo off. Underneath were two T10 and two Philips head screws, both 6-32 UNC. I have replaced them with four identical Pozidriv screws. Another half hour should get the fragments of calibration labels off my fingers, chair, and lab carpet. I also hate people who have not heard of "minimum bend radius" and cables; the 2m lead had been tightly bent and held with a huge tie-wrap.

Does it work? Yes, it does. Phew!
Does it have stunning CMRR? Yes, it does. Hurray!
Does the "Offset" control work properly? Possibly, although there seems to be some 60Hz breakthrough. Given that I'm in a 50Hz country, I'm assuming that's coming from the TDS3032's display. I'll check further later. There might be some duff capacitors...

First impressions: It's a lot bigger than I expected; about the size of a tall paperback. It's heavier, too; the case is thick folded steel. The TekProbe interface not only powers the ADA 400A, but also tells the oscilloscope what range is in use. Because the switches on the ADA400A are genuine mechanical, you have to use both the oscilloscope's vertical control and the preamplifier switches. Later stuff (like the TCPA 300 current probe) can control all gain from the 'scope. It's a bit noisy (41uV RMS) with its bandwidth set to 1MHz and 'scope bandwidth set to 20MHz, but that's not really what it's about; what you're paying for is that superb CMRR. I can see it being very useful.

Further investigation will follow...

[EC8010]

I had a poke around and found that one of its LT1027-5 5V references was only producing -4V, and noisy at that. Amazingly, I had an LT1027-5 in stock. Admittedly in the old TO5 package, but with care its leads went into the 8-pin DIL holes. It's now a proper goer! I looked up the list price of an ADA 400A at Farnell (it's still available); £3,360 plus VAT! Even importing it from Eire (and being done for VAT and duty), I paid less than one tenth of that. As Kryten said, "Engage smug mode."

xdevs.com investigated an ADA 400A and theirs had a faulty LT1021 in the +5V reference. Looks like this is a problem to watch out for.

Mine didn't appear to have been interfered with before, but it was pretty shoddily assembled with DIL ICs poorly aligned in their holes and poor soldering; I suspect hand assembly on a Friday afternoon.

BEWARE! According to a note on https://web.archive.org/web/20250319171 ... x/ada400a/, changing gain whilst powered can damage a TPA-BNC adapter. Very naughty if true.