INA333 industrial amplifier - strange behavior - Page 1
Forum section: electronics / beginners

author: den
author_rank: Contributor
timestamp: August 02, 2020, 03:25:05 pm
content: Hi all,I'm trying to make a simple industrial (as per AoE) amplifier based on INA333. I need to amplify a signal from a load cell, which will have 10 mV output max (only to positive side). I have no split supplies, the amp will be supplied from 5 V rail. I am using a 200 Ohm gain setting resistor, which results in gain of 501. Currently I am testing it with thermocouple (the easiest thing I could think of for creating a few mV of input signal). The setup is seen below in the picture. I connect the minus of thermocouple to a voltage divider so that it's slightly above ground (in this case 380 mV). The REF pin is connected to ground. My current status - the output is limited to 760 mV, and then clips. Up to this level (1.5 mV input) everything look fine - gain is correct. I have also tried to connect the REF pin to 1 V and to 2.5 V (as per datasheet) - just using simple voltage dividers for that (but this should no be a problem here I think). In each case my output offsets by the REF voltage, but still clips at REF+750 mV.As some of you have definitely noticed the output clips at voltage twice the offset voltage. So I have checked if that's the case - with 190 mV offset voltage output also clips at 380 (or so) mV. Conclusion - my output clips at twice the offset voltage. But why? What am I doing wrong? Maybe I have fried the chip while soldering?

author: magic
author_rank: Super Contributor
timestamp: August 02, 2020, 04:07:50 pm
content: You are doing everything right, your expectations are wrongLook at the internal schematic. Pins 1 and 8 are maintained at the same potential as 2 and 3 by A1 and A2. A voltage equal to the input voltage appears across Rg. The 50kΩ resistors cause A1 and A2 to amplify the input signal - A1 amplifies the positive side "upwards", A2 amplifies the negative side "downwards". At some point A2 hits the negative rail and the circuit no longer works correctly.

author: den
author_rank: Contributor
timestamp: August 02, 2020, 07:14:37 pm
content: Thanks for the answer magic.I think I got your point, but - I did try to put 2.5 V on the REF pin, that should have solved that problem, right? But it did not - I had a max Vout = 2.5+0.76 = 3.26 V ! My signal was over 4 mV, which should have resulted in an output of 4.5 V, but I had only 3.26 as mentioned. The only thing is - I used a voltage divider for the REF bias, but as far as I understand - it decreased transient performance (hence CMRR), but for the "static" signal it should not matter..?I have managed to attach a schematic to my first post, maybe one can see an error there...

author: wraper
author_rank: Unknown
timestamp: August 02, 2020, 07:22:07 pm
content: Quote from: den on August 02, 2020, 07:14:37 pmLook at it's schematic. Ref input has nothing to do with input op-amps.

author: magic
author_rank: Super Contributor
timestamp: August 02, 2020, 08:35:37 pm
content: CMRR is also a thing at DC. If the supply voltage changes, the REF voltage will also change. Just connect REF to ground.editWTF, I'm retarded. That's PSRR. But it's true neverthelessSimply put: if decreasing the input voltage to 190mV decreases the maximum output voltage to 380mV, what will happen if you increase the input voltage to 2.5V?

author: den
author_rank: Contributor
timestamp: August 03, 2020, 06:01:37 pm
content: So should I just bias my input to 2.5 V? I can try that, but I don't thing it's mentioned anywhere in the datasheet. The dependency on input bias generally is not mentioned anywhere. I think I have connected everything according to datasheet, but the thing does not work, that's what bothers me.@wraper - I was just referring to datasheet - it states that "The output REF pin is connected to mid-supply. Zero differential input voltage demands an output voltage of mid-supply".

author: magic
author_rank: Super Contributor
timestamp: August 03, 2020, 07:27:39 pm
content: Quote from: magic on August 02, 2020, 04:07:50 pmIt really is that simpleThe internal schematic on the first page explains why it must be so. For a simple example, considerVin-=2VVin+=3VRg=25kΩand calculate what are the output voltages of amplifiers A1 and A2. You will be able to see why it wouldn't work if you change it toVin-=1VVin+=2VRg=25kΩ

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 03, 2020, 07:38:06 pm
content: Quote from: den on August 03, 2020, 06:01:37 pmYou need a 'minus' power rail. Or maybe other design.

author: magic
author_rank: Super Contributor
timestamp: August 03, 2020, 07:52:36 pm
content: No negative rail required unless the input is centered around ground and can't be any other way. Input common mode voltage must simply be in the middle between the negative and positive supply.For a single +5V supply system, figure 21 in the datasheet shows the acceptable input common mode range depending on desired peak output voltage.

author: eblc1388
author_rank: Frequent Contributor
timestamp: August 04, 2020, 06:33:41 am
content: Quote from: den on August 03, 2020, 06:01:37 pmMagic is correct. The datasheet did mention the input common mode range depends on the output of amplifier A1 & A2.

author: den
author_rank: Contributor
timestamp: August 04, 2020, 08:19:54 pm
content: Seems that I can directly use the schematic from figure 35 from the datasheet. They put 1.5 V to REF pin, I guess just because force transducer can produce negative voltages too, but in my case it is very uncommon.I'll try offsetting the input by 2.5 V, but now I have few doubts that it will work. Thanks for the help!