digital pot instead of analog pot - Page 1
Forum section: electronics / projects

author: brownt
author_rank: Regular Contributor
timestamp: June 14, 2018, 07:23:28 am
content: Hi,I would like to use a digital pot in place of the analog pot in this circuit. However, it does not work properly. There are some very tiny sound fluctuations but that is all. I wonder what needs to be done, or why its not working as expected, how I can work out what the problem is.thanks

author: mikerj
author_rank: Super Contributor
timestamp: June 14, 2018, 07:47:16 am
content: It would help if you told us which digital pot you used, and ideally a schematic showing how you wired it up.

author: brownt
author_rank: Regular Contributor
timestamp: June 14, 2018, 09:02:02 am
content: Hi, its a microchip digital pot. like mcp4361Wired just like the analog pot is wired. two ends and a wiper. Perhaps I am missing something there, as the questions seems pointless.

author: mikerj
author_rank: Super Contributor
timestamp: June 14, 2018, 10:19:48 am
content: Quote from: brownt on June 14, 2018, 09:02:02 amDid you bother looking at the datasheet? Hint: What is the min and max voltage permitted on the wiper pins?

author: brownt
author_rank: Regular Contributor
timestamp: June 14, 2018, 11:04:08 am
content: The voltage on the pins must stay within the power supply range of the digital pot, so in this instance 0 - 5 volts.There is no DC on the pot as there are caps on either side. So, do I need to measure the amplitude of the AC, and have a positive & negative supply for the dig pot within that amplitude?

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 14, 2018, 11:28:56 am
content: An easier method would be to bias the grounded end of the pot to + 2.5 V. As both the other terminals of the pot are AC-coupled, this DC bias will not affect the rest of the circuit.

author: alanb
author_rank: Frequent Contributor
timestamp: June 14, 2018, 12:54:02 pm
content: Is the digital pot linear whereas the analogue pot was logarithmic?

author: Gibson486
author_rank: Frequent Contributor
timestamp: June 14, 2018, 01:04:42 pm
content: So, you move and change the digipot, the bias changes as well. This is fine with a regular pot, bit with a digipots, you need to mindful of where the 0 signal is referenced or you risk going beyond the rails. For your application, there are better digipots available, but it will cost you, however, for the point of learning, this is a good first step.NVM...it is biased at 0V. So yeah, you may be clipping on the lower end.

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2018, 08:23:39 am
content: If the digital potentiometer is biased at 2.5V, then another capacitor is required on the output, to block the 2.5V DC.Here's an example of how it can be done. The 0V node of the potentiometer is biased at 2.5V, via a potential divider and AC coupled to 0V. As shown the bias is taken from the 9V supply, but it might be better to change the 39k and 15k resistors to both the same value, say 10k, and take it from the 5V rail.

author: Alex_MGM
author_rank: Newbie
timestamp: June 22, 2018, 09:38:53 am
content: I used this very simple schematic. It works very well but it's operation depends on transistor used.I tried several ones and it turned out that the best (for this application) are very cheap Russian (and Soviet, olad stock) KT315. Similar is for example BC847B.

author: Zero999
author_rank: Super Contributor
timestamp: June 22, 2018, 10:38:50 am
content: Quote from: Alex_MGM on June 22, 2018, 09:38:53 amThat won't be very linear and will cause some distortion. It's also not a digital potentiometer, unless it's connected to the output of a DAC, but a crude voltage controlled resistor. J-FETs make better voltage controlled resistors, than BJTs, because they're more linear.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 22, 2018, 01:22:17 pm
content: Quote from: brownt on June 14, 2018, 07:23:28 amThe pot needs to be controlled by a serial data link. I don't see that in the schematic?

author: Zero999
author_rank: Super Contributor
timestamp: June 22, 2018, 01:52:28 pm
content: Quote from: ahbushnell on June 22, 2018, 01:22:17 pmThat's probably because only the analogue part of the circuit is shown. There will be a microcontroller, but it isn't relevant to the question which was about using a digital potentiometer, in an analogue circuit.

author: Alex_MGM
author_rank: Newbie
timestamp: June 22, 2018, 02:09:25 pm
content: In my post I only offered regulating element for electronir resistor, even method of regulations.I tested it and it works rather good.It is possible to use FET transistor instead og bipolar, there are different ways to apply regulating voltage to base of the transistor. You may use mechanical potentiometer of DAC capability of microcontroller. But method of regulation seems very elegant to me.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 22, 2018, 03:31:18 pm
content: Depends on how he is doing ot if it is an issue.Sent from my SM-G930V using Tapatalk