We are designing a new DAQ for measuring output from Hemispherical Resonator Gyroscope sensor.DAQ should meet the requirement of measuring charge of order 0.001pC at 5KHZ frequency.Could you please confirm the solution based on CN0350 evaluation kit meets our requirement of measuring charge in order of femto coloumb.
What is the range of input charge that you will be measuring? The total system error of the CN0350 after calibration is at 0.25% FSR. To be able to attain accurate measurements of 1 fC, capacitor C2 should be changed such that the input charge range is much less than 400 fC. A good design rule is to make it less than half of this range if you want to detect 1 fC changes and less than a tenth of this range if you want to accurately measure 0.001 fC.
Below is the equation used to calculate C2 for a sensitivity of 1 pC/g and 1000g range making the input charge range to 1000pC.
The Range of input charge is 0.001pC to 3pC
The range of input charge is too wide. Even without considering the input noise of the op amps and using the full ADC input voltage swing of 0.1 to 2.4V, the minimum ADC input voltage to detect 0.001pC for a 3pC input charge range is 333uV. However, the AD7091R only has a 12-bit resolution which limits its LSB to 610uV.
My suggestion is to use a higher resolution ADC such as the AD7124 (https://www.analog.com/en/products/ad7124-4.html) which has 24-bit resolution. You can use the same signal conditioning as the CN0350 with a different Ccal value (1.3pF at most for a 0.1 to 2.4V swing) in a single-ended connection with any of its analog inputs. You can choose to use the same reference which is compatible with this ADC or use its internal reference through the REFOUT pin. The AD7124 is also for ultra-low power applications.
Thanks for your reply. & suggestion of alternative ADC.
Maximum input charge for DAQ is +/-3pC and minimum it should measure is 0.001pC.
C2 value would be around 2.7pF with a gain of 1 for second stage op-amp for ADC with resolution of 13bits or higher.
Considering 0.25% total system error for range of 3pC,then the error would be 0.0075pC and would not be able to measure charge in range of 0.001pC. Please confirm.
Please confirm the above calculation of system error in case of 3pC input charge.
Your computation for C2 and the system error is correct. The system error arises from multiple sources such as temperature drift of the resistors and voltage reference; and offset drifts from the AD8608.
I have got CN3050 EValuation kit and tried to calibrate using function generator with 1KHZ sine wave of amplitude 1V as per the reference document.The output of FG is connected to CAL and NEG signals of J1 connector.After calibration for 1000pC,I tried to acquire the signals with same FG and the following observations were made:
There is ripple overriding the sine wave and also the frequency of acquired signal is 50Hz which is equal to line frequency and the ripple frequency is 1Khz.
If I change the amplitude of sine wave also there is no change in MIN and MAX coulomb values.
Could you please provide your inputs to resolve this