LT1101 weirdness - Page 1
Forum section: electronics / projects

author: IO390
author_rank: Regular Contributor
timestamp: July 13, 2014, 06:19:17 pm
content: My current project needs a good instrumentation amp with a gain of 10. This is for amplifying a DC voltage of 0-100mV up to 0-1V. After searching around, I decided to have a play with the LT1101 as no external resistors are required.So I connect it up as per the datasheet, with the following pinouts:1 - GND2 - GND3 - GND4 - GND5 - +5V6 - signal7 - pin 88 - outputI then applied the 100mV into pin 6 and the output was about 800mV. This is a bit odd, seeing that I was expecting a gain of 10. I then increased the supply to 12V or so, and increased the input to 500mV. Again, there was an error as the output was 4.2V.I've fiddled around with it, but I cannot remove the error. I've looked through the datasheet and can't find anything that explains the problem. Am I just being stupid and missing something obvious?ThanksIOP.S. I bought two amps, I've tried both and they both present the same error, so the problem must be with my configuration.

author: Marco
author_rank: Super Contributor
timestamp: July 13, 2014, 07:24:57 pm
content: Is the inverting input allowed to be at the negative rails?

author: KerryW
author_rank: Regular Contributor
timestamp: July 13, 2014, 09:08:26 pm
content: I think Marco nailed it. In LTSpice, it needs at least 25mV on pin 3 in order to give an accurate output voltage.I ran in to a similar phenomenon with an LT1990 recently. It would be nice if they mentioned that in the spec sheet.

author: IO390
author_rank: Regular Contributor
timestamp: July 13, 2014, 09:56:18 pm
content: I tried again with the negative supply at -1V and it works perfectly. I've got a -1V rail in the project anyway so the other opamps can swing to ground, so this no extra work to implement.Thanks for the help guys!

author: Marco
author_rank: Super Contributor
timestamp: July 14, 2014, 02:36:54 pm
content: Quote from: KerryW on July 13, 2014, 09:08:26 pmWell if you think about it it's kinda natural for a BJT input opamp ... this one uses PNPs so it won't be limited by Vbe near the negative rail but it still needs some Vce to work with.Only JFET opamps can really be expected to work close or even a little below the negative rail, JFETs being depletion mode devices.

author: dannyf
author_rank: Super Contributor
timestamp: July 14, 2014, 03:17:52 pm
content: QuoteThe datasheet should have an item called input voltage range or common mode voltage range.Generally speaking, an opamp with pnp input pairs can swing to the negative rail (input-wise). An opamp with npn pairs can swing to the positive pair.In your case, you can use a R2RI opamp, an opamp like LM358, or introduce a common mode voltage (going negative rail or level shift the input, ...)....

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2014, 05:56:48 pm
content: Quote from: dannyf on July 14, 2014, 03:17:52 pmOffhand the only NPN input operational amplifier I know of with an input range that includes the positive supply is the old LM301/LM301A. Are there others which are *not* rail-to-rail input designs?That feature of the LM301/LM301A was not widely advertised but it did come in handy before rail-to-rail input operational amplifiers became ubiquitous. I have looked for others like it since it is useful for high accuracy current sensing on the positive supply but never found any.