Input offset voltage on AD7616 is 2.5V and is hard to pull it down to GND

Hi, ucuk .

Please see the following insights:

1.) What is your signal source? Since you have stated that it might exceed 21V. Also, do you put a signal at the VxA_M pin?

2.) Please see the image below:

    2.1) Where does the +5V_EXT go?
    2.2) I noticed the orientation of your target LPF. Are you targeting to also put a LPF to the VxA_M inputs? If so, then your orientation should be the suggested circuit.
    2.3) Do you have a

3.) I tried simulating your current circuit vs. the suggested circuit. Your circuit does not seem to cut off the beyond 15 kHz. The red graph is your circuit. The blue graph is the suggested circuit.

Warm regards,
Jo

Hello NathanT 
1. So input is connected to CN2 which is terminal block, i have +5V (internal step up converter to power from single li-ion). Input source is external sensor with voltage output in max range +-10V usually 0-5V.
Idea is to create logger with 16channels and be able to power few small current drawing devices like presure sensor (with voltage output) or flow sensor - also voltage output, but supply from 24V (also created from single li-ion) and here is risk that someone short terminal block pin 24V to INPUT and give. Here is my whole input schematic - but this should not be relevant

2.1) +5V_EXT is my power supply created on board from step up converter to power external sensors.

2.2-3) I was using reference schematic from datasheet, and eval board. I really want to have reference GND connected to GND because GND will be power supply of my external sensors. Exactly like in datasheet AD7616 below.

3) Yes, you are right, this may be wrongly calculated, thank you for pointing this out, but you connected signal souce from other side :) I will connect to CN2/3/5 so capacitor is after resistors.

But this is completly side topic. My problem is that internal vref 2.5V is pulling my floating input to 2.5V. Floating means that i disconnected for test R59,R60. C40.

As i can understand this is due to low input resistance (1M) and input opamp configuration - sems to be substracting.

BR,

Tomasz

Hi, ucuk .

Apologies for misunderstanding your diagram. I just want to clarify your statement, "When both inputs are floating then i have correct 0V reading from ADC and difference between V0A and V0AGND is 0V so reading is correct." What is the difference of this setup compared to when floating, as in removing all the components at the input?

I have some assumptions for the 2.5V. Let us try narrowing it down. Please see the following insights:

1. When floating the inputs, can you try putting a probe to measure the voltage across  V0A? Let us check whether the 2.5V exists.

2. Try pulling the REFSEL to LOW. This makes the device disable the internal 2.5V. Let's check whether the 2.5V reading still exist.

3. Do you read the 2.5V all the time? Or just upon powering up the device?

4. Do you read the 2.5V to all channels?

Regards,
Jo

Hi NathanT 
I've made schametic to explain what i mean floating. I measure this voltage (red) by external multimeter.

1)I made measurement and placed them on schematic

2)I pulled REFSEL to GND over 10k, and now - voltage is reading not stable and instead of 5V i got ~7.3V. Floating voltage drop to 1.533V (from 2.154V). Measured voltage on CASE 2 is 1.2821v - i'm not providing external 2.5v ref so this is expected as i understand

3)yes

4)yes

I made exact measurement, they are marked on schematic below. I made typo in floating voltage previously. Measured floating viltage is 2.154V. This is marked on CASE1

CASE 2 - this is my PCB design and my schematic. 1.818v directly on V0A and on input connector (terminal block). also there is visible voltage drop across Bottom R. this is around 20mV. this give around 2uA - which i can confirm using uA multimeter. When i short terminal block input to GND there is current around 1.67uA

I made short video what is going on. not connected input (CASE 2 from schematic above). When i connect nothing i can see 2V. touching it can change slightly. when i connect short to GND or short to +5V then voltage reading is perfect.

On display you can see 4 channels, so i'm dealing with IN0 marked as C01. Voltage reading is ok - it was calibrated with external supply so there is conversion is software.

Hi NathanT 
I've made schametic to explain what i mean floating. I measure this voltage (red) by external multimeter.

1)I made measurement and placed them on schematic

2)I pulled REFSEL to GND over 10k, and now - voltage is reading not stable and instead of 5V i got ~7.3V. Floating voltage drop to 1.533V (from 2.154V). Measured voltage on CASE 2 is 1.2821v - i'm not providing external 2.5v ref so this is expected as i understand

3)yes

4)yes

I made exact measurement, they are marked on schematic below. I made typo in floating voltage previously. Measured floating viltage is 2.154V. This is marked on CASE1

CASE 2 - this is my PCB design and my schematic. 1.818v directly on V0A and on input connector (terminal block). also there is visible voltage drop across Bottom R. this is around 20mV. this give around 2uA - which i can confirm using uA multimeter. When i short terminal block input to GND there is current around 1.67uA

I made short video what is going on. not connected input (CASE 2 from schematic above). When i connect nothing i can see 2V. touching it can change slightly. when i connect short to GND or short to +5V then voltage reading is perfect.

On display you can see 4 channels, so i'm dealing with IN0 marked as C01. Voltage reading is ok - it was calibrated with external supply so there is conversion is software.