switching opamp design to single supply - Page 1
Forum section: electronics / projects

author: vince76
author_rank: Newbie
timestamp: August 21, 2018, 08:56:17 am
content: Hi all, I'm new here.I was looking around to find some circuits to build an AC constant current source.The circuit is needed to inject current into samples of different materials and take resistivity measurements.I found the circuit in the attachment and I was wondering if it's too complicated changing from dual supply to a single supply.The signal gen at the input will be replaced to an 20-50 Hz oscillator, probably built using an opamp with single suply too.the current needed at the output will be variable from few mA to say few hundred milliamps. The sample under test is represented in the circuit by R6.The reason of the supply conversion is the option to have portable device operated with battery.Suggestion about different designs are welcomeThankyou for you help,Vince

author: Dave_C
author_rank: Contributor
timestamp: August 21, 2018, 10:18:10 am
content: Hi!What kinds of resistances do you want to measure?If they are really low (less than 1 Ohm for example) For IC2 you will have to use RRIO opamp, IC1 will either have to be a high output current type or have an external buffer at the output. If you have to use certain Opamp that can't operate near the ground you can generate negative supply rails using switched capacitor converter like 7660 (Datasheet from TI:https://www.ti.com/lit/ds/symlink/lmc7660.pdf) the chip will generate negative voltage equal to positive supply that you have ( assuming it's no more than 10V )If they are really high ( more than 10k Ohms ) you will have to use higher supply voltages for the opamps or reduce the current ( at 1mA voltage across 10k Ohms is 10V - your supply voltage needs to be higher than that, see output voltage swing in datasheet of any opamp you want to use )

author: vince76
author_rank: Newbie
timestamp: August 21, 2018, 10:23:50 am
content: hi, thankyouI missed the information about resistance range.I think we are talking about tenth of Ohms to some thousands of Ohmsthe Vcc I was thinking is 50V

author: Dave_C
author_rank: Contributor
timestamp: August 21, 2018, 10:42:59 am
content: It's going to be hard to find good opamps for 50V supplyOne that I found is LM675 (http://www.ti.com/lit/ds/symlink/lm675.pdf) for driver opamp (don't remember the number anymore)For the sense Opamp use ADA4522 (http://www.analog.com/media/en/technical-documentation/data-sheets/ada4522-1_4522-2_4522-4.pdf) - It has rail-to-rail output, and input common mode range includes ground.Both of these are quite expensive and hard to get.Feel free to search for other opamps and you might find better ones.It all depends on the required accuracy.As for the supply, look for Power Opamps with >50V max supply voltages, and you need one with high current output, and one with rail-to-rail output and preferably input.

author: vince76
author_rank: Newbie
timestamp: August 21, 2018, 10:46:56 am
content: opa547 ?http://www.ti.com/lit/ds/symlink/opa547.pdfBut still remains the answer: would it be possible to design a non complicated single supply for that circuit?

author: Dave_C
author_rank: Contributor
timestamp: August 21, 2018, 10:59:39 am
content: Opa547 is also ok for this roleFor the supply you can even use an off the shelf power supply ( 48V is a common output voltage)If you need precise output voltage and low noise you can use a linear regulator like LM317HV (the HV at the just means it's a high voltage variant Datasheet:http://www.ti.com/lit/ds/symlink/lm317hv.pdf).