[Pmod AD1 Troubleshoot] Reading Error

[lukelouyu]

Hi, everyone here.

Now, I have tried with my Pmod AD1. I used the demo code provided by the Digilent, but the value shown in the Vitis Serial Terminal is different from the value I have seen in the multimeter. 

For example, when I insert my sensor A into the channel A0, the value shown in 2.56. The value shown in the multimeter is 2.9V.

When I insert my sensor B into the channel A1, the value shown in  the Vitis Serial Terminal is 1.08. The value shown in the multimeter is about 2.1V.

When the sensor induced voltage exceeds 1.8V, the AD1 value returns back to 0. 

I think this diagram might tell me the reason. But I am not sure since I am new to FPGA & electronic and I don't quite understand the datasheet. (Link: https://www.analog.com/media/cn/technical-documentation/evaluation-documentation/AD7476A_7477A_7478A.pdf?_ga=2.196220652.1995249494.1621070558-1887244704.1616928603)

Through comparing with the AD1 reading voltage and the multimeter voltage, i found there is an linear relationship between them. I made an excel file to show you guys the relationship. 

 

I suspected i need to add an additional voltage source but not sure as well. When I connects the Vcc to the D0 or D1, the serial terminal said 3.3V. I did not change anything in the demo code, is there anyway to allow the AD1 reading to be the same as the reading in the multimeter. 

 

Thanks

Luke

Edited May 15, 2021 by lukelouyu

[JColvin]

Hi @lukelouyu,

I will take a look into this and let you know what I learn.

Thanks,
JColvin

[lukelouyu]

@JColvin

Thanks. Btw, I also changed the ref voltage to 1.8 and get this information.

 

[zygot]

ADC datasheets are tricky to navigate even for seasoned engineers so don't feel bad about being a bit confused.

Most ADC converters, and the successive-approximation type like the AD7476A are mixed signal components; they have a digital section for sending the output code to another digital device, and an analog section that connects to the circuit being measured. The part of the datasheet that you've posted refers to the digital part of the converter and isn't likely to be related to your problems.

It will help if you tell us what sensors you are connecting to the AD1 analog inputs.

Trying to get your ADC to agree with a multi-meter may or may not be a good idea depending on the sensor output.

Edited May 18, 2021 by zygot

[lukelouyu]

@zygot

I am currently using DF robot gravity EC sensor and pH sensors

Here are the links for these two sensors:

https://www.dfrobot.com/product-1123.html (EC sensor)

https://www.dfrobot.com/product-1782.html (pH sensor)

Here are design specification:

Thanks

Luke

[zygot]

I took a glance at the information available for your sensors. Both use an interface board to create a voltage output with some relationship to the transducer output. Unfortunately, not much information about either the conversion board or the transducers for either sensor is provided.

There are two major things to consider here. One is sensor output impedance and ADC ( including analog driver circuitry ) input impedance. Ideally, the sensor would have very low output impedance and the ADC very high input impedance. The other consideration is whether the sensor output is constant or fluctuating. The description of the sensors that I saw, and I only briefly skimmed through what was provided, indicates that it is not constant.

A multi-meter display reports the results of some average measurement, usually under 20 readings per second. So, for a constant signal it can be pretty accurate. For a fluctuating signal the display can be very misleading.

If you aren't well informed about the basics of conversion between the analog and digital domains you might think that any ADC module will work with any signal that meets a limited set of specifications, like voltage range. Unfortunately, this isn't, by and large true. The details that one needs to consider aren't necessarily obvious from the component datasheets and specifications; sometimes the datasheets and specifications that are published conveniently ignore the details that you need.

I doubt that the AD1 is the correct converter that you need but choosing the right one takes a bit of investigation.. if you know what you are looking for,

ADI has some good application notes and books available on the subject for scientists that aren't converter specialists.

Edited May 18, 2021 by zygot