Is OPA277 a good op amp for an electronic DC Load? - Page 1
Forum section: electronics / beginners

author: Prithul0218
author_rank: Contributor
timestamp: January 17, 2019, 12:18:07 pm
content: I am going to make a simple electronic DC load. It will be digital. I am probably going to use MCP4725 as the DAC and ADS1115 as the ADC. (cause these are available as module cheaply from China).I will be using OPA277 to make a simple voltage sense circuit. I have a few of these OPA277s, so I was thinking if I could use it as the main MOSFET driver op amp as well. If not, suggest me decent through hole and fairly cheap op amp that will do a good job.Feel free to suggest me anything else for the project.Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2019, 12:41:34 pm
content: It seems reasonable. The only issue might be its inputs need to be 2V away from either supply rail, for it to work properly. Refer to page 8 of the datasheet: Common-Mode Voltage Range.http://www.ti.com/lit/ds/symlink/opa277.pdfYou've not posted a schematic, so there's no way of telling whether this is a problem or not.Care needs to be taken with frequency compensation, to avoid oscillation, but this is true, irrespective of the choice of op-amp.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2019, 12:57:31 pm
content: The strength of the OPA277 is more with a relatively low input bias and low offset. If there is a low offset, one might be better off with a lower noise, that has more of a bias, like OP27 or similar. If cheap is priority OP07 could be a good choice.

author: Prithul0218
author_rank: Contributor
timestamp: January 17, 2019, 02:19:17 pm
content: Quote from: Zero999 on January 17, 2019, 12:41:34 pmThanks. I have not designed the schematic yet. I have just made a simple LCD UI code and tested it. I will eventually select all the parts and start making the schematic. But here's a schematic that Scullcom Hobby Electronics made. I will use it as reference as I am just a hobbyist.I don't have an oscilloscope and can not afford one right now. So, I can't really do much about the noise. I'll ignore it for now I guess. So other than that, OPA277 seems like a good candidate?

author: Prithul0218
author_rank: Contributor
timestamp: January 17, 2019, 02:23:22 pm
content: Quote from: Kleinstein on January 17, 2019, 12:57:31 pmNow I am getting a little bit confused. I already have OPA277 for the sense circuit that I'm going to copy from the schematic I attached in my reply above. I can afford OP27/OP07.I do care about precision/accuracy. I will use one op amp as voltage follower for the current sense ADC input. Like U7D in the schematic above.So, which one should I go for? OPA277(already have at home)/OP27/OP07?

author: Paul Rose
author_rank: Regular Contributor
timestamp: January 17, 2019, 02:43:06 pm
content: Is this thishttp://www.scullcom.uk/category/projects/dc-load/the project your are using for inspiration?It is using an AD8630 in its driver with a positive supply of 5 volts and a negative supply at ground.The AD8630 is rated for "5 V single-supply operation". But the OPA277 requires inputs to be 2 volts inside the supply voltages ( 2v below v+ and 2v above v- ).If you try to run OPA277 from single supply 5 volts, your input would have to be in the range +2 to +3 volts.Ithinkthe final version of that project did use OPA277 in its volage sense amplifier, but it generated a -5v rail for its negative supply.

author: Prithul0218
author_rank: Contributor
timestamp: January 17, 2019, 02:53:18 pm
content: Quote from: Paul Rose on January 17, 2019, 02:43:06 pmYes! I'm using that as inspiration. I will use higher voltage since OPA277 can handle it. I will be using a transformer to generate 12V,5V,-5V rails. and 4.096V, -2.5V reference. for current nulling. Same way Louis Scully did.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2019, 03:02:36 pm
content: The suitable OPs depend on the supply and the use in the circuit. The OPA277, OP07 and OP27 all need a negative supply.The buffer for the cheap DAC is not that critical - here it's more the question if one needs one at all. So the OP0 would be well good enough here.The critical OP usually is the one amplifying the low voltage signal from the shunt. This could be the same OP driving the MOSFET, but could also be an amplifier stage first.The OPA277 normally is rather expensive, usually more than the OP27 and way more than an OP07.The OP27 is rather fast and could thus tend a little more towards oscillation.With no scope, it would be a good idea to first simulate the circuit. One might also use a simple AC detector to at least get an idea of the amplitude of possible AC signals.

author: Prithul0218
author_rank: Contributor
timestamp: January 17, 2019, 03:34:13 pm
content: Quote from: Kleinstein on January 17, 2019, 03:02:36 pmSince I already have a few OPA277, it's actually less costly for me to use that then buying OP07 (and also waiting for the slow China posts).So, maybe I should go with OPA277?

author: exe
author_rank: Unknown
timestamp: January 17, 2019, 03:39:46 pm
content: I'd start with requirements. Chances are, a cheap opamp like tl072 or lm358 will do the job (the latter one can go quite close to negative rail).