How can Rpi read ADS1256's negative values?

Question

I'm using this Raspberry Pi High-Precision AD/DA Expansion Board from waveshare. I have some questions regarding its ADC (model name: ADS1256) unable to give out negative value. Specifically, only giving out negative values around -0.1V. Here are details of my problem.

Current situation:

1. Using the user manual provided with the board, I have managed to read analog input data from on-board potentiometer and LDR (converted to digital data) using their sample codes. In order to use the on-board potentiometer and LDR, I've connected jumpers to the appropriate positions as metioned in the manual.

2. Now, I've connected a function generator, sending out +/-2V amplitude of analog sine wave signal, to the waveshare board.

3. By modifying the sample code provided, I've printed out the converted data and ploted them, which is the figure shown below (y-axis = voltage, x-axis = number of data)

• figure 1: sine wave plot

Here are the questions:

1. Is this normal phenomenon for this ADC board?

2. If so, am I correct to assume that this ADC can only receive analog inputs above zero? (I've looked through the ADS1256 datasheet and studied a bit, but had hard time understanding the input range found in the document, shown below.)

3. If above assumption is true, then can the analog input range be changed?

• figure 2: datasheet screenshots, page 2 and page 3

Any kind of help or comment would be greatly appreciated. Thanks!

Product page: https://www.waveshare.com/wiki/High-Precision_AD/DA_Board

Manual : https://www.waveshare.com/w/upload/b/b7/High-Precision-AD-DA-User-Manual.pdf

Sample codes : https://www.waveshare.com/wiki/File:High-Precision-AD-DA-Board-Code.7z

ADC datasheet : https://www.waveshare.com/wiki/File:ADS1256.pdf

Answer 1

am I correct to assume that this ADC can only receive analog inputs above zero?

Well, the datasheet says the analog input range is + - (Vref * 2) / PGA. In other words, if you set Vref = +2.5V, PGA = 1, then the input range is +-5V.

Though you can input +-5V, what is measured depends on Vrefp and Vrefn, as described in the overview.

Answer 2

Your ADC isn't working because you're exceeding the ABSOLUTE MAXIMUM RATINGS listed in your question:

Analog inputs to AGND: −0.3 to AVDD + 0.3 (V)

In other words, this ADC cannot measure negative voltages. Most simple ADCs can't do that.

You could overcome the problem by adding a constant offset voltage to your signal (google "Non-Inverting Summing Amplifier"), then subtract this known offset from the ADC measurement in software. Or, if electronics is not your strong side, you could simply put the signal through a resistor divider attached to AVDD: a 2x divider will let you measure voltage from +AVDD to -AVDD:

^{simulate this circuit – Schematic created using CircuitLab}