minimum input voltage in sigle rail mode (datasheet only talks dual) - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 12:03:09 pm
content: I'm looking at the attached datasheet of a chopper compensated opamp. Basicaly i need to read a reference voltage (that varies) and generate a voltage slightly higher and slightly lower as a range for which a test part is measured against.The datasheet bangs on about all specs with a supply of +/-5V even though in it's headline it states single rail operation, my gut feeling is that to work properly with input voltages of a few mV it needs to have a negative supply. There is also precious little talked about specs above 5V as it can go to 8V so even powering it at 6V might well give me better output range to get full 5V output and not 4.7V (although not thst important).Basically I need to read a reference sensor and then add or suntract (for which i bet I need negative voltage anyway....) a small voltage, that I can then compare my sensor under test to the range for a pass/fail test.

author: seriouscoinage
author_rank: Contributor
timestamp: October 07, 2015, 12:17:30 pm
content: The datasheet says that the common mode input voltage range includes the negative rail VDD-. This means that if you connect the negative rail to ground, the opamp inputs can swing all the way to ground and it will work fine. It can also work on a single-supply voltage of up to 16V (although that's the absolute maximum,) which is the same thing as being able to run on +/-8V. However, one thing to keep in mind is that the output will never quite reach 0V on a single supply. The datasheet claims that it will reach at least -4.7V on a +/-5V supply, which is the same thing as it reaching at least as low as 0.3V on a 10V supply. If you need the output to reach ground, then you will need a negative supply (although it doesn't have to be large - 0.5V would do.)

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 12:22:27 pm
content: Well I'll essentially have to use it as a summing amplifier, to get my input minus a fixed voltage i need to supply a negative voltage so sounds like i need a negative rail as even the positive offset voltage will only be a few mV so too close.

author: seriouscoinage
author_rank: Contributor
timestamp: October 07, 2015, 12:27:04 pm
content: Could you post the circuit you're planning to use?

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 12:30:53 pm
content: I'm still dreaming it up.Basically I need to do a window comparator operation to compair two sensor assemblies. So i use a known good sensor as a reference, both the test sensor and the reference sensor will be stressed together in exactly the same way. So i need to use a window comparator to compare my test to my reference that varies, so I need to take my variable reference and then produce two outputs say Vref+5mV and Vreff-5mV to use as references for the window caparator.

author: seriouscoinage
author_rank: Contributor
timestamp: October 07, 2015, 12:37:26 pm
content: You might try adding an identical constant voltage to both of your sensor inputs, so that the outputs are both above ground, but their differential voltage is the same. What kind of sensors are they?

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 12:39:14 pm
content: pressure sensors, they produce 0.3-4.7V output so it's all positive voltage operation but because I need to ass and subtract a few mV I am also working around the 0V

author: seriouscoinage
author_rank: Contributor
timestamp: October 07, 2015, 12:47:22 pm
content: A differential opamp amplifier would let you subtract 5mV without doing anything too close to ground. I can't think of a circuit which would let you add 5mV easily, though.

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 12:49:45 pm
content: would that be by using two different gains ? so I subtract say 0.5V but divide it by 100 with the gain resistors to actually subtract 5mV.that would remove the need for a negative rail.

author: seriouscoinage
author_rank: Contributor
timestamp: October 07, 2015, 12:54:42 pm
content: In fact, what you could do is this: Subtract 5mV from both Vref and Vsense (voltage of the sensor being tested.) Then using your comparators, test if Vsense < Vref-5mv and Vsense-5mV > Vref (the same as testing Vsense > Vref +5mV.) If either one of those conditions is true, then Vsense varies from Vref by more than 5mV, but you only need 2 single-supply diff amps to get the input voltages. (At least, I think this should work...)

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 12:56:28 pm
content: Quote from: seriouscoinage on October 07, 2015, 12:54:42 pmexcellent idea. Just looking up differential opamp amplifiers.

author: seriouscoinage
author_rank: Contributor
timestamp: October 07, 2015, 01:11:16 pm
content: I don't think the standard differential amplifier circuit will be able to subtract 5mV using gain ratios and an e.g. 0.5V reference - the gain ratios must be matched for it to work as a proper differential amplifier. You will need some way of supplying a constant 5mV to the inverting input of the differential amplifier, unless there's something I'm missing.

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 01:15:23 pm
content: The easiest way of doing the test is to compare a known good transducer to a test one. So the transducers will both be presurized and their outputs compared, so I need to create a window around the reference sensor output to compare the test one to.Acording to the formula I found the subtracting voltage can be controlled with a resistor ration but one of those also matters in the non inverting voltage input stange so i may need to play my values right.

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 01:44:36 pm
content: nope, can't be done with a diff amp, as both gains get affected.

author: Simon
author_rank: Unknown
timestamp: October 07, 2015, 04:46:41 pm
content: Hm, further complications, there is a potential 5% of full scale error that also needs nulling out (it will be nulled out in the application), something like a sample and hold circuit or by now it might just be worth using a microcontroller and a differential amplifer to measure the difference and amplify it so that I get good resolution with my ADC (and possibly over sample).If I do it all analog I needDual supply (ok may need this anyway),negative and positive voltage references (or a chopper op amp to invert the positve reference)Sample and hold circuit with really long stupid hold to measure starting offset and hold it during test. and because I want low input offset I'd end up making it with a chopper opamp.two differential chopper amplifiers to create the two references that the pass window falls inComparatorsMotor PWM for the pumpBearing in mind that I have 3 sensors to measure that are of 2 types and want to use two reference sensor for each making that 4 and with no uC it will be a very mechanical process to run the test instead of a preferable push button and wait for lights.Much of this doable in a uC if I use a summing amplifier for my two reference sensors, Use a diff amp to get the initial offset that needs nulling out and amplify it to get more AD resolution (10x) although I need to deal with negative results to ofset the result. Maybe a second set of diff amps with higher gain to do the actual in measurement comparison (that won't work as there will still be the real analog offset there).All in all complext both ways but the uC allows for more automation and bells and wistles plus I've not actually looked at the sample and hold circuit yet that may need to hold a value for some seconds and take some seconds to store a value assuming it's even feasable (anyone ever made a sample and hold with a super cap)