More Op-Amp Nulling - Page 1
Forum section: electronics / beginners

author: ed_reardon
author_rank: Regular Contributor
timestamp: February 27, 2017, 07:37:24 pm
content: Hi all,I'm working on a current to mV converter (4-20mA loop powered) for a pressure transducer.I have the 'loop' across the instrument with an op-amp acting a differential amp across a 1ohm precision resistor (low-side), that far I have got, the next stage is gain which I'm okay with it's getting rid of the offset that's giving me the headache.When I have 4mA flowing in the circuit I'm getting fairly close (for my purposes) 4mV on the output of the amp, however when there is 4mA flowing the circuit I need to have near 0V output on the amp, then to increase by one mV for every mA. (ie. 4mA = 0mV, 5mA = 1mV, 20mA = 16mV etc.)My question I guess is would it be acceptable to use the input null of something similar to a 741 to offset a differential amp, or am I better off building another circuit to do this?Cheers as always,Ed

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2017, 08:07:44 pm
content: The OPs zero adjustment should not be used to compensate much more than the OPs own offset voltage. Especially with BJT based OPs like the 741, the offset drift depends on the offset adjustment and is small only if the adjustment is about right.One usually uses a resistor larger than 1 Ohms for the conversion. So more like 50 or 100 ohms, so one can directly interface with an ADC or similar.

author: rstofer
author_rank: Super Contributor
timestamp: February 27, 2017, 08:15:18 pm
content: It sounds a lot like "offset and scale" to me! Chapter 4 here:http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

author: ed_reardon
author_rank: Regular Contributor
timestamp: February 27, 2017, 08:17:58 pm
content: Thank you Kleinstien, I don't want an mV output that exceeds more than 20mV, so I'm quite happy to work down in the mV range.I need a full scale output of 0-30mV from a 4-20mV input signal.

author: ed_reardon
author_rank: Regular Contributor
timestamp: February 27, 2017, 09:45:01 pm
content: Does this make any semblance of sense?It's just asketchrather than a finished schematic obviously, and I'm rubbish at designing them on computer so it's in pencil I'm afraid!

author: Benta
author_rank: Super Contributor
timestamp: February 27, 2017, 10:25:39 pm
content: If you want to get a 0 V output, you'll have to provide a negative supply voltage as well.From your sketch, you're powering your amps from +36 V and 0 V, which won't work.If you power your circuit from, say, +30 and -5 V, cancelling out the 4 mA is dead easy.You can inject a constant voltage, for instance.My solution of choice would be bleeding 4 mA from the input to -5 V through a (trimmable) current sink. This eliminates offset/differential problems in the amps.

author: retrolefty
author_rank: Super Contributor
timestamp: February 27, 2017, 10:38:24 pm
content: One of the main features for the 4-20ma current loop 'standard' is the concept of having a 'live zero', where a any value under 4 ma is considered a invalid measurement value, perhaps from an open circuit in the loop. Most industrial 4-20 current loops measure the loop value by routing the loop through a 250 ohm resistor to gain a 1-5vdc analog measurement. Any offset and of course scaling to engineering units can then be best done in firmware/software.

author: ed_reardon
author_rank: Regular Contributor
timestamp: February 28, 2017, 07:16:32 am
content: That's the idea, this instrument will remain in a 4-20mA current loop, I need to piggy-back a small circuit to provide a 0-30mV output for another device not in the current loop.I would use an micro/DAC however obviously the whole power usage of this parallel circuit must not exceed 4-mA

author: Sigmoid
author_rank: Frequent Contributor
timestamp: February 28, 2017, 01:16:29 pm
content: Actually, I've gotten rail to rail op amps in single supply mode to give me 0V. You just need to put a diode or two on the output (BEFORE the feedback) so the minimum output voltage is lost in the diode drop. I'm not sure if I'm angering some greek god of opamps with this circuit, but it worked for me so far.

author: rstofer
author_rank: Super Contributor
timestamp: February 28, 2017, 02:05:18 pm
content: Quote from: ed_reardon on February 27, 2017, 09:45:01 pmDon't the 2 op amps feeding the output op amp need summing resistors to match the output stage feedback resistor?

author: Brutte
author_rank: Frequent Contributor
timestamp: February 28, 2017, 07:40:57 pm
content: Unreadable schematic.

author: ed_reardon
author_rank: Regular Contributor
timestamp: March 01, 2017, 11:54:25 am
content: Yes, yes they do!I'll have another bite at this later!Ed

author: ed_reardon
author_rank: Regular Contributor
timestamp: March 14, 2017, 08:30:47 pm
content: Sorry it's me again.Notwithstanding the awful sketch above, would the trick of using a voltage divider across an op-amp non-inverting with the rail voltage to create a 0V point on the output work.Then using the circuit with reference to that to create a 0V -> -18V rail supply for the offset amp?

author: rstofer
author_rank: Super Contributor
timestamp: March 14, 2017, 08:51:39 pm
content: Sure, you just add -4 mV (if I remember the question).The problem is, you don't have any precision reference for the 4 mV. As I see it, you allow for a wide range of supply voltage and if the supply voltage varies, so does the offset voltage.It is usually assumed in all these designs that the power supply is stable at some known voltage and even then, we sometimes use a voltage reference diode (or chip) to create the voltage. That voltage may be divided as necessary but at one end of the divider chain, the voltage is constant and precise.http://www.ti.com/lit/ds/symlink/ref102.pdfI don't know if there is a negative 10V reference. If not, just run the 10V through another dual supply op amp to get -10V.

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2017, 09:10:29 pm
content: I remember looking at this thread weeks ago!If you're hoping to power this thing off the 36V 4-20ma as in Reply#7, surely everyone can see at least 1 big problem.

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2017, 09:31:17 pm
content: LOLPerhaps you could use a 3.3V to 5Vshunt regulator(TL431 ?), powering a high side current sense IC (0.5mA ?), with the 4-20mA current sense resistor now just above the shunt regulator.I only had a quick look, but you'd still need a few mV of-VEon the IC's ref to offset the 4mA to near 0V on the CSIC's output.