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Problems with using chip CDC AD7745/46

Question asked by Mikhail on Aug 21, 2015
Latest reply on Aug 26, 2015 by EricC

Hello all!

We have some problems with using chip CDC AD7745/46. We develop a pressure transducer with a metal differential capacitive sensor. Characteristics of sensor: С0х = С0у =135 ± ΔС pF, where С0х, С0у - static capacity of the sensor`s bridge branch, ΔСmax = 55 pF - dynamic range of variation.

After researching of capacitance measuring methods our team made conclude. Under these conditions optimal is method of capacitance conversion by Σ-Δ ADC with extended dynamic range as instructed "CN-0129/Extending the Capacitive Input Range of the AD7745/AD7746".

We mountig a model of measuring channel. The dynamic range is extended equal to 28. We made imitator of differential sensor based on the variable capacitor. The parameters of the sensor`s imitator: С = const = 97 pF; С = 91+76 pF.

After writing of the necessary value into control registers the model are measuring the capacitance on full dynamic range of measure. We have received such codes: minimal 8388608 (it's for zero of sensor`s capacitance in differential mode), maximal 16777216 (this value may be less depending on the settings of the gain register). At the same time a channel of the temperature measure is setting built-in sensor and working normal (T=31...32 °С).

If we configure a channel of the temperature measuring with using of external temperature sensor (it's thermistor or semiconducting sensor) we have increase of the capacitance`s channel value by 3.5-4 times. This effect occurs independently of the reference voltage source used into a channel of the temperature measuring - built-in or external.

The mechanism of a crosstalk between the capacitance`s channel and the temperature`s channel are not describe in the specification CDC AD7745/46. So we would be grateful for the any help in this matter. This is important for us because we will use the external temperature sensor in the pressure transducer (it will measure the temperature of pressure sensor).

We found some singularity. If we change the settings of the excitation voltage`s control register (we change EXC_A+, EXC_B- to EXC_A-, EXC_B+), the AD7745/46 is inverts the signal measuring the capacitance. This is explained by the functional of the Σ-Δ-ADC, but the CDC AD7745/46 specifications have no such information.

We ask you to provide information:

  1. 1. What a reason we have a corruption of capacitance`s channel value when we measuring a temperature by the external sensor? How decide this problem?
  2. 2. Is the inversion of the signal measuring the capacitance (if we change the settings of the excitation voltage`s control register) the normal functioning of CDC AD7745/46?

Thank you for your answer!