_pA89
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Can anybody (continue to) answer this thread posted @ADIEngineerZone
I created once a range extension circuitry for the AD7143 (eight channels, single excitation signal) based on the considerations in [1]. The "bulk" capacitance was ~100 pF, which was compensated by the internal OFFSET register (-> set to a zero reading), as I was only interested in a capacitance increase. The range extension was designed to be operated up to several hundrets of pF, but in the end it has only shown a linear relation in the lower range (+ ~200 pF).1. How (much) Scaling Factor F reduces Accuracy of Measurement. (does it?), Can Scaling Factor be made arbitrarily high?
No, therefore you are calculating your scaling factor which leads to a reduction of your excitation signal amplitude. As you reduce the excitation amplitude, you can increase the capacitance as \[Q = C \cdot V\]. This is the basic idea behind the method described in [1], and is mentioned in section "Circuit Discription" in the beginning of page 3.3. Scaling 9pF capacitor into 4pF range producing negative capacitance Results (CAPDAC=0) (does it?)
This is quite tricky due to the realization of the CDC. As the excitation signal frequency has to be consistent with the internal switching which is performed due to the4. Why Scaling cant be done by External Excitation Signal as pointed out in this
-> The Results gets negative when you just interchange R1, R2 Resistors ? (I understood Q=CV, is the Device Ratio metric [1] or is it sensitive to Excitation voltage Level?)No, therefore you are calculating your scaling factor which leads to a reduction of your excitation signal amplitude.
True, You are effectively inverting effective Excitation Voltage.Interchanging R1 and R2 is equivalent to inverting effective Excitation voltage.
+ Need to understand operating Principle -any mathematical model ("processing" - is what needs to be understood) of AD7745 - CDC Operation (e.g. Ratio Metric method is used for Unknown Resistor Measurement Operation).Due to phase sensitive processing of capacitor current ...
Yes, but that wasn't your initial question. When sacling 9 pF into 4 pF, following the description in AN-1585, the result should be positive.-> The Results gets negative when you just interchange R1, R2 Resistors ?
this article [3] gives a good insight in the working principle.+ Need to understand operating Principle -any mathematical model ("processing" - is what needs to be understood) of AD7745 - CDC Operation (e.g. Ratio Metric method is used for Unknown Resistor Measurement Operation).
As the CDC employs a \[\sum\Delta\] topology it performs an oversampling respectively a decimation. The sample frequency is 32 kHz, so at each 32.25 µs a sample is taken. Due to the operation principle, a decimation is performed which is an averaging process. This reduces the final output/update data rate. Have a look on page 14 in [4], where this is mentioned for the AD7143.+ ECX-Excitation Frequency(Signal Frequency) = 32KHz, Update Rate = [10,90] Hz (Nyquist Criterion)?
It's a square wave, as this signal is used to charge your capacitor of interest.1. [3] doesn't mention any Excitation Source (Square Wave)- EXCA?
A \[\sum \Delta\] ADC samples/compares the signal at a high frequency (called sample frequency fs), which is a multiple of the output-data frequency fd times the decimation rate DR (or oversampling rate OSR). Have a look at [5]. The \[\sum \Delta\] modulator provides a high bit stream with a (sample) frequency fs = fd • DR. This bit stream is then filtered/decimated and results in the output-data rate fd = fs/DR.2. Consider PCM1802 - Audio ∑Δ - ADC
The DR/OSR for the AD7745 is not stated. But according to [4] page 14, for the AD7143 the sample frequency should be 250 kHz (as the excitation frequency) to hold the stated equation (3 • 256 • 1/(250000) = 3.072 ms). Thus, the sample frequency for the AD7745 should be 32 kHz. Note, the unknown capacitor (sensor) is charged/discharged at this rate.* f_oversampling = 64*f_sampling || 128*f_sampling (Page-11)-------------f_oversampling_AD7745 =?
The AD7745 is a CDC and not a classical ADC, so the implementation of a filter structure is affecting your sensor/capacitve reading. Have a look on other CDCs from ADI as e.g. the AD7143 provides a shielding option, which might help to minimice EMC issues.3. Also the Author in this article (hackaday.io -22-bit-capacitance-to-digital-converter) points out disadvantages of AD7745: (Any Comments)
- Susceptible to EM interference because there is no input filtering.
- It's not ratiometric. It relies on a 1.17V internal voltage reference.
That -detailed analysis- could shed some light into internal operation of AD7745 IC.To determine the influence of this drift would require a more detailed analysis.
Interested in Accuracy of AD7745 when Device is Scaled - Not Mentioned in Datasheet. (e.g. Resistors can be scaled Linearly - Connecting them in Series, Non-Linearly - Connecting them in Parallel, to bring their value within range of interest)The AD7745 states an ENOB of 21 Bit, so the question is what resolution do you need?
Sorry, I do not get the non-linear parallel configuration of the resistors. Are you talking about the TCR? If so this also affects the series configuration of resistors. Further, the TCR is usually stated with \[\pm\], so it also might happen that two resistors in series or parallel might compensate each other as one might show a temperature dependent resistance increase and the other one a decrease. If you are conserned about the TCR, use one with a low ppm/°C and estimate its influence on the excitation signal amplitude. Use thin film resistors, as these show good TCR linearity and low noise, further they are available with a low (initial) tolerance.Interested in Accuracy of AD7745 when Device is Scaled - Not Mentioned in Datasheet. (e.g. Resistors can be scaled Linearly - Connecting them in Series, Non-Linearly - Connecting them in Parallel, to bring their value within range of interest)
Can we Safely Assume AD7745 also employs similar Mechanisms of MAS6510 (So that we can do Accuracy Calculations Theoretically), Equation Given Below for MAS6510 sheds some light for that IC."You might use a calibration structure enabling..."
I thought it would be irrelevant to main conversation and the image shared is not edible for public in general!please share your considerations here in the forum instead of sending a private massage
Exactly what is described in Pg-3 of AN-1585 - Application Note -Characteristics of Capacitive Humidity SensorFor me it is still not clear what are you trying to achive!?
AN-1585 figure 2.Citation needed