ADS1291: Data convert to mV

Hi,

Before injecting ECG signal/data, have you tried the internal test signal?

Also, may I ask whether you have an ADS1291(2) evaluation kit/board(EVM)?

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May I ask where/how do you get the equation? 

Please follow datasheet 8.3.8 Data Format and Table 10. Ideal Output Code versus Input Signal to convert.

"The LSB has a weight of VREF / (2^23 – 1)."

If you inject a differential input signal, i.e. INPUT SIGNAL, VIN (AINP – AINN) = 0V or short, do you see ADC output code around 000000h?

AND,

If you inject a differential input signal, i.e. INPUT SIGNAL, VIN (AINP – AINN) = your Vref, do you see ADC output code around 7FFFFFh?

It's always recommended to use internal tests signal to validate before injecting any external signal.

In the meantime, you may also take a look of this thread-

https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/772488/faq-ads129x-how-do-i-convert-adc-output-codes-to-volts/2856893#2856893

Thanks

Thank you for your reply.

I saw that equation in the link below.

https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/673869/ads1291-data-format-convert-to-mv

1LSB = (Full Scale Range (2*VRef))/2^n = (2*2.42)/2^24

1LSB / Gain

Calculate the expression above and divide the gain.

And then I multiplied the ECG data.

My VREF = 4.033, Gain = 12

So ECG Data * (2* 4.033 / (2*24)) / 12) came out.

As you said, datasheet 8.3.8 says, "The LSB has a weight of VREF / (2^23 – 1)." Why is he calculating like that?

Thank you.

Hi,

Data sheet -              "The LSB has a weight of VREF / (2^23 – 1)."

Other engineer     1LSB = (Full Scale Range (2*VRef))/2^n

where, n is 24 bits for ADS1291.

If you divide the equation by 2/2, then you will get to very similar to the datasheet equation. I guess other engineer may have off-by-1 error/mistake.

I suggest to stay with datasheet.

Thanks

Before injecting ECG signal/data, have you tried the internal test signal?

Also, may I ask whether you have an ADS1291(2) evaluation kit/board(EVM)?

Hi.

Thank you for replying.

Is there any problem if I pay using the calculation formula below?

• The calculation formula is based on the calculation formula of the link questioner below.
• https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/673869/ads1291-data-format-convert-to-mv

Vecg / Gain = (4.033v * 20000 / 32767.0) / 12 = 0.2V

• 4.033v = vREF,  20000 = ECG_Data,  32767.0 = Maximum value of my ECG data,   12 = Gain

And since I need mV, multiply by 1e3.

And if I use a filter, it comes out like the picture below.

The x-axis is the mV and the y-axis is the data length.

Is this okay?

Thank you.

Hi.

I don't have evaluation kit/board (EVM) and I didn't put in a test signal.

Thank you

Hi,

0.2V for a ECG signal might be too large. Is that what you are injecting?

May I ask where/how does the "32767" come from? 

ADA1291 is a 24-bit ADC.

Why didn't you use the equation in datasheet? 

Data sheet -              "The LSB has a weight of VREF / (2^23 – 1)."

Thanks

Hi.

Thank you for your reply.

ChienChun Yang said:

0.2V for a ECG signal might be too large. Is that what you are injecting?

• Because it's too big, I multiplied it by 1e3 to make it an mV

ChienChun Yang said:

May I ask where/how does the "32767" come from? 

• The maximum was 8388607 but I changed Data Read Commands 
  
  • RDATAC to RDATA
• I think 2^15 - 1 = 32767 But I don't know why it changed to 2^15 - 1, can it change?
• Does the LSB change because Data Read Commands change?

I'm measuring on my wrist right now, and I know the ECG ranges from -2mV to +2mV.

And I know ECG * LSB = ECG unit V should come out.

• ECG = Data Received from ADS1291
• ECG unit V = output

LSB = VREF / (2^23 – 1) as per datasheet.

ECG * LSB = ECG unit V

The average of my data is 20000.

Then 20000 * VREF / (2^23 – 1) = 0.010192347787898516V

Is this the right formula? Even if I multiply by 10^3(1e3), the value of 10.19234 comes out.

I don't know if this formula is correct.

Thank you.

Hi,

No sure I understand what you mean by "Because it's too big, I multiplied it by 1e3 to make it an mV"?

Do you want to clarify?

For, "I think 2^15 - 1 = 32767 But I don't know why it changed to 2^15 - 1, can it change?"

Where do you get 15 from?

"Then 20000 * VREF / (2^23 – 1) = 0.010192347787898516V" this makes more sense and looks about right.

 What might be you Gain setting? You may want to divide the value above by the Gain setting.

For, "Even if I multiply by 10^3(1e3)"

Again, Could you explain why you want/need to multiply 10^3?

Thanks