MCP3551 - Single reference - Page 1
Forum section: electronics / projects

author: SpottedDick
author_rank: Regular Contributor
timestamp: May 07, 2024, 11:53:30 am
content: Hi,I had a post on this already, but I had everything completely wrong and so muddied the waters and wanted to start again.I'm looking to measure a 0-2.5V signal using an MCP3551.MCP3551 Data sheet:https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/20001950F.pdfBlock diagram attached.I'm trying to lower down the component count.With a 0-2.5V signal, can I can tie both Vin- and Vref together to a 1.25V reference to get the full range?VDD is 3V and VSS is 0V.If understand the operation correctly, Vref is basically the swing voltage and Vin- is the "centre". So with this setup, I should get the full 22 bits from 0-2.5V. With 0-1.25V being negative and 1.25V to 2.5V being positive. Is this correct?Just as a bonus question, I'm not sure how linear the output is going to be. For example, does the 0-1.25V range have 21 bits spread across it equally?

author: magic
author_rank: Super Contributor
timestamp: May 07, 2024, 09:00:20 pm
content: I think you understand it right.As for linearity, figures 2-1 to 2-3 appear to plot measurement error (in parts per million of full scale input range, 2·Vref) vs applied differential input voltage. Note that all INL specs apply to the case of common mode input voltage being Vref/2, but CMRR is typically 135dB so CM voltage shouldn't make a huge difference hopefully.

author: kevin.gibbs
author_rank: Regular Contributor
timestamp: June 27, 2024, 03:35:37 pm
content: To get the full range, it is most correct to make Vref = 2.5V. According to point 3.2 of the datasheet, Vin = Vin+ - Vin-.

author: SpottedDick
author_rank: Regular Contributor
timestamp: July 12, 2025, 12:49:27 pm
content: I completely missed this response and I'm planning to come back to this project soon, so trying to refresh myself on things I was stuck on.QuoteSo I do understand what I was planning here.QuoteNow this response has confused me. Did you mean to write (Vref = ((Vin+)-(Vin-)))?

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2025, 02:41:40 pm
content: It depends on the Vcc (2.7V to 5V).With your 3V Vcc you can use 1.65V Vref max and you will get "22bit resolution" as well.For 2.5V Vref you would need 5V Vcc.Connecting Vref to Vin- creates the "virtual ground for the input source" at Vin-.The Vcc/Vss has to be "floating" against your input voltage source then.

author: SpottedDick
author_rank: Regular Contributor
timestamp: July 12, 2025, 03:32:35 pm
content: Wow, I think I must need to go back to bed or something today, so sorry if I seem like the densest person in the world at the moment!QuoteCan you expand on this? I don't understand what this means.Long after I made this original post, the actual circuit was made. The circuit gets a signal that starts at 1.2V. I haven't got the spreadsheet with me today to check what the actual max input is, but lets just say it's 2.4V.Here's the circuit for clarity:Questions:1. Will the measurable range start at 1.2V?2. What is the max measurable voltage?SD

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2025, 04:18:25 pm
content: When you connect Vref=+1.2V with Vin- (your plan) your input voltage range will be from -Vref to +Vref MEASURED AGAINST Vin-.Thus Vin- will become the GROUND for the input signal and Vin+ will be the INPUT for your input signal.

author: SpottedDick
author_rank: Regular Contributor
timestamp: July 12, 2025, 05:36:07 pm
content: Can you try and answer these questions? If you do I might understand better what's going on:1. Will the measurable range start at 1.2V?2. What is the max measurable voltage?

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2025, 06:17:31 pm
content: Quote from: SpottedDick on July 12, 2025, 05:36:07 pmIn case you connect Vin- with Vref=+1.2V while the GND for your input signal will be node Vn- and input of your input signal will be at node Vn+ (and your Vcc=3V):1. no2. +1.2V

author: magic
author_rank: Super Contributor
timestamp: July 12, 2025, 07:58:36 pm
content: Read section 3.2 of the datasheet. The chip measures Vin, defined as Vin+ - Vin-, and compares it against Vref, accepting a range from -Vref to +Vref.Therefore, if Vin- = Vref = 1.2V, Vin+ can vary from 0 to 2.4V and the ADC will read 0V as negative full scale, 1.2V as zero and 2.4V as positive full scale.

author: SpottedDick
author_rank: Regular Contributor
timestamp: July 12, 2025, 08:07:09 pm
content: Quote from: magic on July 12, 2025, 07:58:36 pmThank youSOmuch for clarification. iMo's comments had me really confused. (No shade on iMo, my brain seems to be running at 10% power today).As the first line in this whole thread reads, this is exactly how I wanted it setup (that's a 1.25 reference, it's mislabeled, it's 0-2.5V).

author: iMo
author_rank: Super Contributor
timestamp: July 13, 2025, 10:40:21 am
content: Quote from: SpottedDick on July 12, 2025, 08:07:09 pmBetter to power up the brain at least to 40% and read DS and do the math..