HP/Agilent 35900E 24 bit ADC question/pics - Page 1
Forum section: electronics / testgear

author: PioB
author_rank: Regular Contributor
timestamp: November 14, 2015, 09:50:15 pm
content: Hi,I got an "old" (date codes 94/95) Agilent/HP 35900e for analytical chemistry (HPLC) purposes which contains a 2 channel ADC board. According to the documentationhttp://www.agilent.com/cs/library/usermanuals/Public/G35900-90407.pdfit contains two 24-bit ADC (The analog-to-digital channels provide 24-bit analog-to-digital conversion ofup to two independent analog signals.) I attach pictures of the analog part. The board is multilayer with two ground planes, so by shining light through it, one cannot follow the traces in the analog section. I was trying to find some of the parts online to read the data sheets and learn about the implementation, but either some of the parts were specially created/named for Agilent or my google-fu is seriously lacking.So going from the left, we've got the following:-Valor PT3789http://datasheet.octopart.com/PT3789-Pulse-datasheet-13095801.pdfIsolation transformer, only found this two page datasheet? Power in?-2 times Pulse PE-64103http://www.mouser.com/ds/2/336/pulse_64103-464036.pdfEthernet Isolation transformers. Data out?-Two each of LM7805, LM79L15, 8L15A for regulating the analog rails (5, +/-15 V)-TI 4093V4T Quad 2-Input NAND Schmitt TriggerAnd now to the analog section with the mystery parts-LM311 High speed quad differential comparatorshttp://www.ti.com/lit/ds/symlink/lm311.pdf"capable of driving lamps or relays and switching voltages up to 50 V at 50 mA", Output collector comparator goes to the Schmitt trigger via a 2870 Ohm resistor(to GND?)- OP07 Op amphttp://cds.linear.com/docs/en/datasheet/OP07.pdf-Now what I assume is the ADC, but I cannot find anything: Analog Devices AD1826-1638-Some Voltage References (I assume, labelled VRx on the silk screen) type: LT1826-1778 (also here, google fails me)-2N5116 in a TO-18 can "silicon P-Channel JFETs" with an extremely unhelpful data sheethttp://www.centralsemi.com/get_document.php?cmp=1&mergetype=pd&mergepath=pd&pdf_id=2N5114-5116.PDFIf anyone has any more information on the circuit or the ADC and voltage references, I't be grateful for a hint.Otherwise, there's a switch mode power supply in the box, which seems to come from some HP PC (0950-2384) and a little pcb with a few membrane switches and LEDs for status. The GPIB-board was missing. So I don't think it is even possible to get any data out of the thing.If anyone wants more photos from the board, I can take them tomorrow.I hope the test equipment forum is somewhat correct for this discussion.

author: PioB
author_rank: Regular Contributor
timestamp: November 14, 2015, 09:50:52 pm
content: And an overview of the board, was too big to fit.

author: Someone
author_rank: Super Contributor
timestamp: November 15, 2015, 03:09:02 am
content: Looks like it might have been a special part, similar products have been built with SAR ADCs or delta-sigma so there is no clue in the original function of the board but looking at the specifications the INL and DC characteristics would suggest SAR. Reverse engineering the front end preamps should be interesting, looks like a bootstrapped fet but not much filtering despite the 15Hz? bandwidth.

author: PioB
author_rank: Regular Contributor
timestamp: November 15, 2015, 09:48:05 am
content: Quote from: blueskull on November 15, 2015, 05:11:35 amI was wondering that when I saw the comparator. Somewhere on the forum here (I believe it was in the 3458a repair thread) I saw pictures of a HP board with parts from different manufacturers designated 1826-xxxx so only they know. I don't even know if I can get in there and measure anything useful as I can't connect it to a compute and even then I don't have the necessary software. I'll see if I'll have the time to measure voltages around the VRx parts, that is, if the rails come up correctly.Sent from my SM-N9005 using Tapatalk