AD627 Rg connection - Page 1
Forum section: electronics / projects

author: harerod
author_rank: Frequent Contributor
timestamp: December 14, 2023, 03:14:33 pm
content: Hi,maybe I am a bit blind, but I can't find the internal connections of the Rg pins of this INA.https://www.analog.com/media/en/technical-documentation/data-sheets/AD627.pdfLooking at figure 35: which Rg pin is connected to R1-R2 and which to R3-R4?I couldn't find this information in the datasheet. Fig. 37 also doesn't help with this aspect.I am reviewing a third-party design. However, I don't have ready access to the actual product, e.g. can't just power up and measure.Much obliged, harerodEDIT 240611: Long story short - ADI eventually confirmed that the connections were as hinted by the datasheet.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 14, 2023, 03:22:57 pm
content: fig 35 is a simplified schematic of the ic, RG resistor goes between pin 1 and 8i would expect a 50k ohms value between theses pinsand from pin 1 or pin 8 you have a 100k ohms going to the REF pin and the other on the OUTPUT pin

author: magic
author_rank: Super Contributor
timestamp: December 14, 2023, 03:31:08 pm
content: It looks like they think you don't need to know.Actually, why do you want to know? These are sensitive nodes which normally only see the gain resistor, resistors are nonpolar devices...Figures 44-46 consequently group 1,2 and 3,8 together, but I'm not sure if it means anything. I have seen the same grouping in the unrelated AD620.

author: harerod
author_rank: Frequent Contributor
timestamp: December 14, 2023, 04:44:49 pm
content: Well, I need to know, trust me.The external connection is a bit more than a single resistor and I need to know the potential potentials. The original designer added an n-channel MOSFET in series to Rg to switch gain, I want to make sure that he chose the correct connections for drain and source regarding the expected signal range.

author: moffy
author_rank: Super Contributor
timestamp: December 14, 2023, 08:58:29 pm
content: One of the Rg pins is connected to the REF pin via 100k, try using a multimeter/resistance with the device unpowered and out of circuit.

author: magic
author_rank: Super Contributor
timestamp: December 14, 2023, 09:49:22 pm
content: This works on my AD620.Pin 1 appears to be associated with IN- and measures 44kΩ to OUT, many megs to REF.Pin 8 appears to be associated with IN+ and measures 44kΩ to REF, many megs to OUT.This is in line with expectations.Coincidentally or not, it agrees with pin numbers on the schematic symbol in the datasheet.YMMV. Different chip, different circuit, different topology, different datasheet

author: harerod
author_rank: Frequent Contributor
timestamp: December 15, 2023, 11:50:46 am
content: magic, thank you for your effort. I got heaps of different INA's in store, no AD627 though.I'll make a note of this issue and do measurements, as soon as I can get my paws on that device. In the meantime I have contacted ADI support.For those who are still curious about my motivation: The original designer uses an n-channel MOSFET in series to Rg to switch gain. In this case 5/50. When the internal control loops are in regulation, the Rg-pins will follow -IN and +In respectively. For the lower gain setting, those inputs may drift apart far enough to forward bias the MOSFET's body diode. If done correctly, the whole scheme works good enough for input signals whose differential only goes into one direction.Trivia: Do not confuse ADI AD627 with Burr-Brown OPA627. The latter is a rather decent OPAMP.

author: magic
author_rank: Super Contributor
timestamp: December 15, 2023, 12:33:22 pm
content: I was curious myself, it has never occurred to me to think about such things beforeAny chance a JFET could work instead? No body diode to worry about, will work even with AC.Not sure if their ratio of RDS(off) to RDS(on) is sufficient.

author: harerod
author_rank: Frequent Contributor
timestamp: December 15, 2023, 01:38:28 pm
content: Well, just keep in mind that the Rds_on is in series with the programming resistor. 4k7 in series to an 1mR MOSFET wouldn't affect the gain much. Btw, if precision where a real issue, one would could go for a digitally programmable part.At the moment I am doing archeology - taking a decades old product and refurbishing it for modern production. Lot's of lore and superstition are involved. No documentation, but heaps of "experience", which lead to requirements like "you MUST take this Infinion transitor to switch that 5V/100mA micro relais".Or, one of my favorites, "we need this LiMn cell to maintain the data in the 24LC256". I suppose you get the picture.