Generating a bipolar offset for an op amp - Page 1
Forum section: electronics / beginners

author: netdudeuk
author_rank: Frequent Contributor
timestamp: August 16, 2019, 11:06:35 am
content: HiI have the below circuit (courtesy of Louis, Scullcom Hobby Electronics).It is a work in progress but adjusting RV7 does indeed allow a positive / negative offset to be added to the output.The problem is that I'm using a precision trimmer to do this and even if I used a regular pot, it would probably be a bit hit and miss in terms of what the offset actually is.So, what I would like to do is either drive the junction of C21 / R15 with a DAC or the like (so that I get to say exactly what the output voltage / offset is) or use a regular pot with an ADC to read in the value at that junction.The preferred option would be to do the former. It wouldn't matter if I needed to use a relay to switch the polarity.Any thoughts please ?Thanks

author: cvanc
author_rank: Frequent Contributor
timestamp: August 16, 2019, 02:06:35 pm
content: I'm not quite sure what your goal is?Are you trying to just null this circuit to have a low DC offset? (If yes, consider an op-amp based DC servo loop.)Or are you trying to *modulate* the DC offset like it was a signal?

author: ledtester
author_rank: Super Contributor
timestamp: August 16, 2019, 02:31:20 pm
content: It's for controlling the DC offset of a function generator circuit. I imagine the OP wants to control it digitally.Schematic is attached. Relevant part of the circuit is bottom left of center.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: August 16, 2019, 02:44:51 pm
content: Quote from: ledtester on August 16, 2019, 02:31:20 pmThanks guys.Yes, that's where I have taken that block of hardware from and yes, I would like to drive it digitally.However, I am using my own DDS device.

author: MarkF
author_rank: Super Contributor
timestamp: August 16, 2019, 02:56:00 pm
content: I found that I could replace the adjustment pot and series with just a DAC and an op-amp to shift the +4V output to +/-2V output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2019, 03:21:13 pm
content: Sure, wire a DAC to it, that's fine. Either use an op-amp to translate the e.g. 0-5V range to +/-9V, or use it directly then bias the noninverting input up to compensate.Mind the 1uF cap affects LF frequency response, which will give sloppy square and triangle waves, and inconsistent amplitude sine waves.Tim

author: netdudeuk
author_rank: Frequent Contributor
timestamp: August 16, 2019, 05:36:39 pm
content: Actually, I have MCP4162-103E/P in stock.https://uk.farnell.com/microchip/mcp4162-103e-p/ic-dgtl-pot-10k-257taps-8-pdip/dp/1840698Could I use this (or a similar digital pot) as a direct replacement for the trimmer ?Thanks

author: MarkF
author_rank: Super Contributor
timestamp: August 16, 2019, 06:58:26 pm
content: I don't see why not WITH the restriction that the voltage on pot terminals A and B MUST be between VSSand VDD.Most likely 0V and +3.3V or +5V depending on the logic levels you are using.You CAN NOT USE -/+9V as shown in the circuit.If you want the -/+9V range, you will need an op-amp to amplify and level shift the MCP4162 output.