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I am testing a self-made ADC shield for Arduino Due based on the ADS1299, a 24-bit ADC with a 24x programmable gain amplifier.

I have a signal generator sketch running on another Arduino Due that can output waveforms via the Due's DAC0 output. I can connect an oscilloscope to the signal generator, and see the waveforms. When I connect the ADS1299 to the DAC directly (at 1x or 24x gain) it can see the waveforms perfectly too.

The problem: I want to use a voltage divider to test the DAC at lower output voltages. To start with, I constructed a voltage divider that reduces the output voltage by a factor of 2. But when I connect the ADS1299 to the voltage divider, it reads 0v (well, noise around 0v).

However, the oscilloscope can see the waveform perfectly when connected to the voltage divider.

What's going on?

Here's the voltage divider circuit I'm using:

Z1 and Z2 are 3.3kΩ. Vin is connected to the DAC output. Vout is connected to either the scope or the ADS1299 (with appropriate grounding in both cases). I've tried the circuit with 2 47Ω resistors too, with the same results.

(I've checked the questions Voltage dividers and ADC's, How to Convert 0 to 10V analog signal to 0 to 2.5V for ADC input?, and Shrinking an input signal for the ADC and some others - it seems like the voltage divider's impedance may be interfering with the ADS1299 sample-and-hold, and the scope's probes may have a way of adjusting for that, but I am not sure. The datasheet says the ADC's DC input impedance is 1000MΩ, and the dividers I tried are not anywhere near that.)

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There could be a couple potential causes for this depending on the magnitude of the DAC output and how this signal is driven to the ADS1299. The DAC may not have the required drive strength capability, so added impedance to the signal path may cause this problem. You may need to buffer the DAC's output.

I have posted your question to TI's E2E forum at the link below. My hope is that other community members will see it and that you can provide more information about your design.

http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/328451.aspx

Best Regards,

Ryan

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    \$\begingroup\$ Ryan - I dont see attribution on TI's site for where you copied the question from. SE does use a creative commons license, and you are free to copy content as long as there is attribution. \$\endgroup\$ – Kortuk Mar 17 '14 at 17:12
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    \$\begingroup\$ Thanks Ryan - this does seem to be at least part of the problem. I added a 1x opamp buffer using an OPA376DBVT, and was able to read the opamp's output using the ADS1299. I can also now divide the opamp's outout by a factor of 2 using a voltage divider and read that reduced output using the ADS1299. \$\endgroup\$ – Adam F Mar 18 '14 at 3:55

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