Getting Around the Positive and Negative Supply for an Op-Amp circuit - Page 1
Forum section: electronics / beginners

author: R-802
author_rank: Contributor
timestamp: June 26, 2025, 06:10:43 am
content: Building a buffer/filter for the CS4272 codec (https://statics.cirrus.com/pubs/proDatasheet/CS4272_F2.pdf), following the recommened circuit in the datasheet but the negative supply is giving me trouble. My questions are:1. Is there some way to get around generating negative supply for the opamps in Figure 12 (attached below)?2. Could we simply power the opamp with 10V on the postive and have the negative going to ground? Is there a specific type of opamp that does this?

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: June 26, 2025, 07:45:13 am
content: what's the trouble?You can use a single supply but then you'd need to design a bias network to center the signal around the bias voltage instead of 0V, but from the schematic, it looks like the buffer circuit already considers that and biases the input to VCOM. It doesn't look like it requires dual supply at all.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: June 26, 2025, 07:54:56 am
content: a quick simulation confirms it:

author: R-802
author_rank: Contributor
timestamp: June 26, 2025, 08:11:38 am
content: Oh, I was just following the schematic on the evaluation board (https://d3uzseaevmutz1.cloudfront.net/pubs/rdDatasheet/CDB4272-2.pdf). Just seemed a bit too simple, guess I can get rid of my 5V analog supply then.

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: June 26, 2025, 08:31:27 am
content: i don't see why using a single supply for those op amps would mean not needing the +5V analog supply. what are you doing? if it's something with just the buffer, you can get away with a single supply if the low level of the biased signal is far enough from ground so that the op amp can swing down to it. many op amps have max swings that are not too close to rail to rail operation(that's probably also why you see they used a+/-18V supply, even if the input is 5.6V max. now seeing at the rest of the circuits in that eval board, it seems the 18V spec is because they needed dual supplies for other things, and so used just that one for all the op amp circuits...), so if your op amp can only get to let's say 1V from the rails, then you want to give it some headroom, so that's something to consider. if your biased signal is far enough from the minimum (and maximum) swing of the op amp with a single supply, then you're good. If whatever you're doing requires dual supplies for another circuit, then just use dual supplies, which again makes me wonder: what's the trouble with using dual supplies?

author: R-802
author_rank: Contributor
timestamp: June 26, 2025, 10:29:21 am
content: The whole 5V analog supply circuit is only for those two opamps, which seems wasteful. Just wanted to know if I needed it or not. I'm building an audio effects processor and the differential signal comes from a single ended to differential conversion circuit. Probably should have explained that eariler, not really familar with this kind of circuit design. This is all I really needed to know:Quote from: Anthocyanina on June 26, 2025, 08:31:27 am

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: June 26, 2025, 11:16:59 am
content: aahh! yeahhappy to have helped, good luck with the project!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 26, 2025, 07:25:53 pm
content: Vcom is 2.5V so you only need +5V and ground to power the opamps.RTFdatasheet

author: RFDx
author_rank: Regular Contributor
timestamp: June 28, 2025, 10:28:01 am
content: Quote from: R-802 on June 26, 2025, 10:29:21 amIt depends on what kind of op amp you intend to use. Look at the input common mode and output voltage range in the respective datasheet. I assume you are using VCOM = 2.5 V (e.g. from the ADC). NE5532 op amps would need a (larger) bipolar supply, no way to avoid that. OPA1656 op amps will get by with a single supply, but need at least 7.25 V DC.The op amps would have to be rail-to-rail IO types in order to power the ADC buffer stage from a single 5 V supply.

author: R-802
author_rank: Contributor
timestamp: June 28, 2025, 12:12:13 pm
content: I thought I’d need rail-to-rail I/O op-amps for a 5V supply, was looking at the OPA1688 (https://www.ti.com/lit/ds/symlink/opa1688.pdf). I figured I could run the input buffer at 10V to improve dynamic range and handle hotter signal levels without distortion.But I recently came across a reference design that uses a 5V supply for the output buffer (since the op-amp's max is 5.5V, THS4521). Now I’m considering standardizing everything to a 5V supply and replacing all OPA1656 with OPA1688. That would simplify the design—VCOM could be buffered with an OPA1688, and both the input and output buffers could also use OPA1688, all running on 5V and GND.

author: RFDx
author_rank: Regular Contributor
timestamp: June 29, 2025, 07:18:02 pm
content: Quote from: R-802 on June 28, 2025, 12:12:13 pmThe OPA1688 has a rail-to-rail output stage. The input common mode range includes ground but is limited on the high side to 2V below the positive rail. Operation up to the positive rail is allowed but you have to put up with "reduced performance" if that happens.

author: davep238
author_rank: Contributor
timestamp: July 02, 2025, 10:00:50 am
content: Page 18 and 19 of the eval board schematics, clearly shows that the NE5532 opamps are powered by +18v and -18 volts.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 04, 2025, 09:35:27 pm
content: Quote from: R-802 on June 26, 2025, 06:10:43 amI don't have an answer for either of your questions, but generating a low-power negative voltage rail can be completed quite easily and cheaply via a charge pump circuit (either based on the NE555, or a dedicated charge pump I.C such as the TC7660) + TL431 shunt voltage regulator. For more complexity, you can create negative voltage rails using switching regulators or modify a cheap switching regulator module.Example 1 circuitandExample 2 circuitshow two ways of creating an NE555 charge pump. They're both similar, but I can't answer the advantages/disadvantages of either.Additionally,here's a Falstad circuitthat you can play around with, based on the Example 2 NE555 charge pump.