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I'm using an ADS1230 for measuring the output of a pressure sensor (MPXM2053). The schematic of the circuit is shown below and some voltages are as follows:

AVDD = 3.304V

DVDD = 3.304V

Common-Mode input = 1.690V

AINP - AINN = 0.0002 V

GAIN is 128 (only R2 is used and R1 is not used)

Schematic of ADS1230 circuit

The problem is that the output of ADS1230 has a large offset. Its typical value is around 389000 (5EF88 hex) while the differential input is 0.0002V. I expect an output around 0.0002 * (2^19 - 1) / (0.5 * Vref / GAIN) = 8134 (1FC6 hex). You can see in the following table the relation between the input signal and the output of the ADS1230 (from datasheet):

Table 4. Ideal Output Code vs Input Signal

A sample of the ADS1230 data (DOUT signal) along with SCLK signal is shown in the following image (SCLK is yellow and DOUT is blue):

DOUT signal along with SCLK signal

I also should mention that I calibrate the ADS1230 after power-on using 26 pulses on SCLK.

What have I done wrong?

Thank you

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  • \$\begingroup\$ The circuit diagram can't be right; with R1 and R2 both being 0R, you are short-circuiting +3.3 to ground. \$\endgroup\$
    – jayben
    Apr 20, 2022 at 8:14
  • \$\begingroup\$ As I mentioned in my question only R2 is used and R1 is not used \$\endgroup\$
    – Kamin
    Apr 20, 2022 at 8:26
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    \$\begingroup\$ The offset may well be an accurate representation of the voltage being output by the sensor. Do you have a voltmeter accurate enough to corroborate what the ADC is measuring? \$\endgroup\$ Apr 20, 2022 at 9:53
  • \$\begingroup\$ @Kubahasn'tforgottenMonica Your point is correct. I need an accurate reference. Previously, I measured it using a [KYORITSO 1009 multimeter ](kew-ltd.co.jp/en/products/detail/00911) and it showed 000.2 mV. It means the the ADS1230 should generate a number near 0.0002*(2^19-1)/(0.5*3.3/128) = 8134 (here Vref =3.3 , GAIN = 128), while it generates a number near 440000 (i.e 10mV) . The accuracy of the multimeter is ±0.6%rdg±4dgt, which I think its error should be less than 1mV. \$\endgroup\$
    – Kamin
    Apr 21, 2022 at 12:02
  • \$\begingroup\$ Later, I measured it using a scope (Analog Discovery 2) and its resolution is 0.32mV. The scope measured the differential voltage equal to 1.4mV while the ads1230 showed a result of 270000(= 6.6mV). Again, it seems the ads1230 has a large offset. \$\endgroup\$
    – Kamin
    Apr 21, 2022 at 12:02

1 Answer 1

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Finally, I decided to change the gain from 128 to 64 by using R1 and removing R2. The result was the same. I doubted the pin connections were correct (I had to do it in the first place). It turned out that the negative reference pin (REFN) was floating.

enter image description here

After fixing it I get the following results which is compatible with the sensor offset range (-0.33mv : +0.33mv @ Vcc = 3.3V) in datasheet:

enter image description here

Result = 10036 -> Voltage = 10036/(2^19-1)*(0.5 * 3.3/128) = 0.00024 V

(Again I'm using R2 hence gain = 128)

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  • \$\begingroup\$ Kamin - Hi, Thanks for coming back with the answer to your question. In order to effectively mark the topic as solved, please consider "accepting" your answer (click the "tick mark" next to your answer, to turn it green). That shows that you don't need more help and future readers can quickly see that this was the confirmed solution. Since you must wait 48 hours after you asked the question before you can accept your own answer, you can accept it any time after 08:04 UTC tomorrow. Thanks. \$\endgroup\$
    – SamGibson
    Apr 21, 2022 at 15:21

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