Original Question:
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.)