ADC - Powering Issue - Page 1
Forum section: electronics / projects

author: Fab64
author_rank: Newbie
timestamp: November 01, 2022, 09:47:56 am
content: I need to measure voltage and current remotely. I am using an ADC1256 off-the-shelf board connected to an ESP32. Voltage can exceed the ADS1256 limits hence I added a voltage follower and resistor divider.Circuit diagram in Figure 1.When the ESP32 is powered with a power bank everything works well.Now I tried to power the ESP32/ADS1256 directly from the dual power supply adding a uA7805 (with a dissipator), Figure 2.Now I am facing these issues:1) Voltage @Tp1 (measured with DMM) changes (e.g. from 0.1942 to 0.16)2) Voltage measured by the ADC is completely wrong (e.g. 0.0032 instead of 0.19)I am not able to figure out what is going wrong. I presume I have a ground issue ....Can anyone help me?

author: jmelson
author_rank: Super Contributor
timestamp: November 01, 2022, 03:49:18 pm
content: Why are you feeding the ADC from a 50 K Ohm source (the two 100K divider resistors)? That is certainly a recipe for poor accuracy at least. Depending on the loading of the inputs by the ADC, you may get very little signal through.Jon

author: Fab64
author_rank: Newbie
timestamp: November 01, 2022, 05:19:00 pm
content: Hi Jon,The max DC voltage allowed by the ADS1256 is 5V so to read up to 15V I have to add the voltage follower (which doesn't drain current from the dut) and the voltage divider (For the initial tests I'm just dividing input voltage by 2).The ADC is configured to have 80M input resistance so the 50K should't be a problem.In fact when ESP32 and ADS1256 are powered using the power bank (hence electrically isolated) the readings are very close to what I measure with the DMM. Anyway, any suggestion to improve precision is very welcome.The real problem arises when I use the voltage regulator. Maybe because the ADC is not isolated anymore from the rest of the circuit. Honestly, I am not able to figure out why.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 01, 2022, 06:27:11 pm
content: You should check that all grounds and 5V lines are connected at the the psu. Dont daisy chain power/ ground you your boards, run separate lines. How does the regulator perform with just a resistor as a load?Check your undisclosed wiring scheme. What ADC board is and where does its 3V3 supply come from. Have you got 3V3?As jmelson says: Driving a line from the junction of this divider sets the source impedance to 50k. An invitation to noise and offset issues. And also messing up the ADCs CMRR.The ADC inputs need to be driven from a low and balanced source impedance.Your DUT looks like a low impedance source, so maybe put the divider before the buffer.OrAdding a few more parts you could use differential signalling. This will mitigate signal ground reference problems. Eminently sensible if you have along line and/ or dirty/ weak ground or power.The ADC1256 will accepts a differential input with all the benfits above. Fully differntial ADC drivers can be configured for gains less than unity. No attenuator needed. You can usually run them on a single 5V supply- this stops you from killing the ADC with an over voltage on an input. Maybe use these at both Tx and Rx ends. Read datasheets for eg AD8139, THS4130 etc.

author: Fab64
author_rank: Newbie
timestamp: November 02, 2022, 06:15:04 am
content: Hi Terry,Thank you for your suggestions.1) 3.3v is used only by the ESP32. Created by an LDO on the board2) The 5.5v used to power the ESP32 is nice and stable3) I will check all the power lines and grounds. Currently everything is on a couple of breadboards and this doesn't help...4) I will try to move the divider before the buffer5) I think that using the differential amplifier could be the best option (let's see if I can buy something in the shortage chip era)