ADS7042: transfer error

Hi Joel,

The customer has a small part of the error in use is very large, beyond the replacement of the converter is normal. They subtract the actual conversion value from the theoretical value and divide by the maximum range. Do you have any good suggestions for this problem?

Input 4~20ma sampling conversion, the above three errors are relatively large, the following one is normal.

The schematic design is as follows:

Thanks~

Hi Rhea,

Can I ask what you are using to generate your 3.3V AVDD source? It could contribute to the noise since the ADS7042 uses AVDD as reference. 

Could I also know what your sampling rate is? This could help me determine if the signal has enough time to settle during acquisition. 

Best,
Joel

Hi Joel,

The customer sample rate is 20ms, and AVDD uses a very accurate 3.3V reference source.

Thanks~

Hi Rhea,

So with a sampling rate of 50 Hz, the signal should have plenty of time to settle before it is sampled, so I'm still unsure of where the error is coming from.

Can you provide more information on the reference that the customer is using to generate AVDD? This will help me make calculations on whether this error is expected or not.

Best,
Joel

您好，乔尔。AVDD用的REF3033AIDBZR生成的3.3V的基准源，不同的AD转换器在同一块板上有不同的输出误差，关于这个误差咱们有一个标准的范围吗？谢谢

Hi Yong,

Here's the translation I gathered from your response:

"Hello, Joel. The REF3033AIDBZR used for AVDD generates a 3.3V reference source. Different ad converters have different output errors on the same board. Do we have a standard range for this error? Thanks"

I see. With a 12-bit device, you shouldn't expect anything worse than a 0.2% error. Let's check if the error you are getting is coming from the ADC or somewhere else in the signal chain. Could you use an accurate multimeter to measure AVDD with respect to ground, AINP (T46) with respect to ground, and tell me what reading you are getting from the ADC in volts? This will be of more use than the error in current.

Best,
Joel

您好，乔尔。

编号1的误差范围相对小一些，编号2、3就较大一些。采样电阻上是有差异性，把编号1的采样电阻放到编号2和3的板子上，误差有所改善，但还是在0.5%以上。编号2更换一个AD转换器，误差就会好很多，把2号的AD转换器换到1号板子上，误差就又差很多。

Hi Yong,

To properly determine the margin of error, you'll have to use something more precise to measure the voltage, like a 6 1/2 digit digital multimeter. You will have to probe AVDD and T46. Along with that, I'll also need the binary output code you are getting from the ADC to see if it matches with what it is seeing at its input terminals. Currently, I can't tell if the conversion which the ADC is outputting is within the specified margin of error. It is okay to try this with only one board for now.

Best,
Joel