The acquisition value is attenuated than the input value in ADS1247 circuit?

[Antony]

Hi,
When I'm debugging ADS1247chips, I find the problem: after I add bias voltage to acquired chip pins of ADS1247, the acquisition value is attenuated than the input value. The datasheet of ADS1247 is http://www.kynix.com/Detail/1250614/ADS1247.html.
AIN0 pin connects a capacitor to the ground. There are (AVDD + AVSS) / 2 bias voltage (2.5 V) as the negative input.
AIN1 pins is positive input.
When AIN1 pin doesn't connect bias voltage, the collection value and the input signal are equal. But when the pin also combines with bias voltage, the acquisition value compared to the input signal attenuates 3 times. Mesuring pin level directly with oscilloscope, for example peak of sine wave is to 2 v, collected value after the conversion of peak value is only about 660 mv.
My application mode is that AIN0 and AIN1 pins have bias voltage. Through blocking capacitance, I input ac signal to AIN1 pin. By the bias voltage, it will be up to 2.5 V, such a wave can be measured waveform ±2 V.

Regards.

[famifan]

no idea, does it have something to do with root-mean-square values?

https://en.wikipedia.org/wiki/Alternating_current#Mathematics_of_AC_voltages
> The dashed line represents the root mean square (RMS) value at about 0.707

are you looking for this?