LTC2422 and external clock

[ggchab]

Hello, I am using an LTC2422 to read the output voltage and current of a power supply (based on Gerry Sweeney's design). To make some experiments, I connected the Fo pin of the ADC to a PWM output pin of the Microchip PIC I am using to control the power supply (18F25K22). When Fo is low or high, no problem, the ADC is working correctly.
To try to maximize the number of conversions per second, I configured the PWM module to produce a 302KHz signal that follows the datasheet specifications (less then 307.2 KHz, high and low periods greater then 0.5µs - verified with an oscilloscope). In that case, the ADC is no longer converting correctly: the EXR bit is almost always set to 1. Even if I decrease the frequency, this is still terrible. What am I doing wrong ? Thank you

[ggchab]

It looks as if a constant is subtracted from the converted value when the Fo pin of the LTC2422 is connected to an external clock. Therefore, when Vin is closed to zero, the converted value is negative. The value increases linearly with the increase of Vin and eventually becomes positive (around Vin = O.025V in my case). When the value is negative, the EXR bit is set to 1 even if Vin > 0.
I don't understand why but, at least, I can use the converted values by simply adding a constant.

[sanwal209]

EXR bit=1 represents that your Vin is out of range. Check your vref voltage and vin voltage level.

Also read the Frequency Rejection Selection (FO Pin Connection) section in datasheet.