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# ADC self-test mode on TMS570LS2125

Hello,

Why the AD conversion result is "approx"?

In the following thread, "There will be some drop due to the resistive pull down to ADRefLo, hence the "approx. ADRefHi" expected conversion result." is described.
However I can't understand what channel is meant for the suggestion.

Could you teach me why "approx" is used concretely?

Best Regards.

Nomoto

• Nomoto,

One of our ADC experts will get back to you on this.

• Nomoto-san,

Vu is the conversion result when ADREFhi is selected as the reference voltage and the channel is converted with self-test mode enabled. In this case, the ADC input will see ADREFhi + ADIN[x] as the input. Suppose the channel x is shorted to ADREFlo. In that case, you have a path from ADREFhi to ADREFlo through the parallel combination of the resistors inside the calibration cell. These resistors are shown as R1 and R2 in figure 19-12 on page 699 of SPNU499b. This will result in a conversion result that is close to ADREFlo, hence the "approx ADREFlo" in the result column for this conversion.

You can apply the same reasoning for the Vd conversion and deduce the reason for the "approx ADREFhi" result.

Regards, Sunil

• Hello Sunil-san,

I suppose that the path is "ADREFlo" -> "a certain amount of register" -> "AD monitor point" -> "R2 // R1" -> "ADREFhi".
Then, "a certain amount of register" is Rmux (max value is 250 ohms).
Is my suggestion correct?
If there is no "a certain amount of register", the conversion result should be "ADREFlo"; because, the path is "ADREFlo" -> "AD monitor point" -> "R2 // R1"  -> "ADREFhi".

2. Could you teach me the resistance value of R1 and R2?
I suppose that "R2 // R1" is approx. 10K ohms.

Best Regards.

Nomoto

• Hello Nomoto-san,

Your deduction is correct. When the input is shorted to ADREFlo and you choose ADREFhi as the reference voltage (conversion of Vu), the path is from the input pin through Rmux, through the parallel combination of R1 and R2 to ADREFhi.

The effective resistance, R1 // R2 is about 3K-ohm.

Regards, Sunil