sensor and voltage divider/opamp configuration - Page 1
Forum section: electronics / beginners

author: peter.andaluz@gmail.com
author_rank: Newbie
timestamp: January 22, 2025, 05:41:48 am
content: I wanted to create a flexible voltagedivider using the fsr402 and an mcp41100digital pot. In this configuration at fullpressure I got a nice range linearresponse.My question is this. The digipot can only take very little current so do I need to take thefsr without RM into a unity gain buffer and then add RM (mcp41100) at the output of theopamp to create a voltage divider that won’t damage the digital pot?Please excuse me if I have broken protocol somehow. I think I have done enough research and experimentation but I need some help with the physical implementation of the circuit.Regards in advance

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 22, 2025, 06:03:53 am
content: I have been designing circuits for almost 50 years but your question is confusing as heck.By "flexible" do you mean "adjustable"?What does pressure have to do with a voltage divider?What is an FSR402?Is RM just a resistor reference designation?The one thing I can say pretty certainly is that the schematic at the top of your page 2 will not work. Generally the current flowing into an op amp input is negligible < 1 microamp (if the op amp is working correctly). Then you will get negligible voltage drop across the digipot regardless of its setting. The voltage at the op amp + terminal will be V+, and Vout will be V+.What op amp will you use and what is the voltage at V+? What voltage will power the op amp?Generally the best way to use a Digipot is as a potentiometer (with some low relatively fixed current end-end and negligible current out of the wiper.) That is the most linear, unless you have some sort of special purpose digipot. Also if you do not need the output voltage to cover the full top to bottom, then it is best to narrow the range with resistors in series with the top and/or bottom ends.

author: ledtester
author_rank: Super Contributor
timestamp: January 22, 2025, 06:18:24 am
content: In your circuit on page 2 the opamp can only output a voltage within its supply rails, so if, for instance, you power the op amp with +5V and 0V, the MCP41100 will be ok.Here is a publication form Interlink on various ways to interface with FSR sensors:FSR Integration Guide - Interlink Electronics.pdfhttps://forum.arduino.cc/uploads/short-url/nnBZahw2y6LDh5G3pBLbPIQMhL2.pdfIs your circuit one of these approaches?

author: peter.andaluz@gmail.com
author_rank: Newbie
timestamp: January 22, 2025, 06:33:08 am
content: Quote from: ledtester on January 22, 2025, 06:18:24 amMy apologies I thought I had provided enough detail as I provided the suggested diagram from the pressure sensor datasheet and on that sheet in the pictures I have included the opamp number mcp6002 by the opamp picture.By "flexible" do you mean "adjustable"? Yes.What does pressure have to do with a voltage divider? A lot as pressure determines the Force Sensing Resistor resistance. No pressure 10Meg aplly pressure and the resistance goes down.What is an FSR402? The FSR designation is Force sensing resistor (FSR)402 is the type. pressure is applied and the resistance decreases. With a voltage divider you can get a near linear response which is what you can see in my chartIs RM just a resistor reference designation? in the picture on page 1 yes RM is a fixed resistor and on page two you can see the outputs for different values of RM. I wanted to have a variable digital pot there in place of RM.The one thing I can say pretty certainly is that the schematic at the top of your page 2 will not work. Generally the current flowing into an op amp input is negligible < 1 microamp (if the op amp is working correctly). Then you will get negligible voltage drop across the digipot regardless of its setting. The voltage at the op amp + terminal will be V+, and Vout will be V+.What op amp will you use and what is the voltage at V+? I have included the opamp number mcp6002 by the opamp picture.What voltage will power the op amp? On Page 1 I have put the voltage in the circuit diagram and the datasheet for the sensor doesn't give a vcc for the opamp but I chose the opamp mcp6002 because it has an output that can get close to vcc so in my circuit 5v power to the opamp.I hope that clarifies your questions and my apologies for not presenting properly.

author: peter.andaluz@gmail.com
author_rank: Newbie
timestamp: January 22, 2025, 06:34:44 am
content: I did my homework and have that document. so can RM in the datasheet be replaced with the digital pot mcp41100?

author: peter.andaluz@gmail.com
author_rank: Newbie
timestamp: January 22, 2025, 06:45:07 am
content: Yes my approach as you can see was from the actual datasheet and their implementation guide.

author: ledtester
author_rank: Super Contributor
timestamp: January 22, 2025, 07:00:24 am
content: Quote from: peter.andaluz@gmail.com on January 22, 2025, 06:45:07 amIn this circuit:I assume you will be operating the digi-pot in "rheostat mode" -- that is, with the wiper connected to either PA0 or PB0. In that case the max current you should allow is 1mA which means the total resistance of the FSR sensor + digi-pot needs to be at least 5K for 5V operation.To be absolutely safe you should not set the digi-pot resistance to anything less than 5K - although you might be able to get away with a lower value based on what you know about the sensor and how much it can get stressed.MCP41XXX datasheet:https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/11195c.pdfYour circuit on page 2 doesn't make any sense and doesn't match any of the circuits in the Interlink document.

author: peter.andaluz@gmail.com
author_rank: Newbie
timestamp: January 22, 2025, 07:13:13 am
content: my circuit on page two was trying to show a way to add a buffer to the digital pot so that I wouldn't destroy the digital pot whilst at lower values.

author: ledtester
author_rank: Super Contributor
timestamp: January 22, 2025, 07:28:55 am
content: You should say what you objective is because some of the other circuits in the Interlink document would probably do what you trying to achieve.

author: MathWizard
author_rank: Super Contributor
timestamp: January 24, 2025, 07:57:38 pm
content: Quote from: peter.andaluz@gmail.com on January 22, 2025, 06:45:07 amIDK about those digipots, or how fast the signals of interest are, but I'd add some small capacitor from the Non-Inv pin to GND or Vcc, just to stabilize that voltage some, and pass higher frequencies to AC GND.And you can always add another current limiting resistor in your divider chain, if you're worried about too much current, it might limit your voltage range, there's a trade off.