Hi everyone - am assuming this is the best forum to post this to given the application.
In my initial design of this circuit the voltage being applied on the left hand side was 3.3V and as a result the design did not include R3 or R4 and the op-amp I was using was the ADI AD8604 with a 3.3V Vin. In that configuration I could reliably read voltage via the AD7997. I've since changed the voltage on the left hand side to 12V which required I replace the AD8604 (12V is beyond its operational range) with the TI LM2902 but because of the 200mV common-mode input range of the CMP393 I want to stay with it which requires I decrease the voltage to something underneath its maximum of 6V. I chose to divide this down to ~2.93V (instead of 5V) and use 3.3V as the Vin for both the ADCMP393 and the AD7997 because the AD7997 wants to drive it's I2C at the same voltage as its Vin and I need this capped at 3.3V for my MCU.
Once I added R3 & R4 I noticed I was no longer getting reliable readings from the ADC, generally speaking my samples were directional accurate but with a wide enough variance that it concerns me and I'm now trying to resolve and improve the resolution. After researching and reading for a few days I decided to post here looking for some advice.
This document https://www.analog.com/en/analog-dialogue/articles/front-end-amp-and-rc-filter-design.html describes the design of an RC filter which I've added to the diagram and enclosed it in the red box. The document describes the method for calculating but it states the input signal frequency, amplitude and ADC conversion time need to be known. The AD7997 has a conversion time of 2uS but I do not know how to identify the input signal frequency or the amplitude and after searching and reading I've not come up with something that tells me how to define this and am looking for assistance with these.
I have placed R3 & R4 both on the IN+ of the TI LM2902 as my schematic shows and on OUT to see if doing so made any difference and I did not see any appreciable difference in my ADC readings.
Regarding Ext R1 & Ext R2 - these are external to my device and one or both of them may or may not be present on a given circuit. If they are present they will most typically be 1K, 2K or 5.6K so the range of resistance coming from them is 0 - 13.4K ohms accounting for 20% tolerances.
Greatly appreciate any guidance or feedback.
thx - sean
In my initial design of this circuit the voltage being applied on the left hand side was 3.3V and as a result the design did not include R3 or R4 and the op-amp I was using was the ADI AD8604 with a 3.3V Vin. In that configuration I could reliably read voltage via the AD7997. I've since changed the voltage on the left hand side to 12V which required I replace the AD8604 (12V is beyond its operational range) with the TI LM2902 but because of the 200mV common-mode input range of the CMP393 I want to stay with it which requires I decrease the voltage to something underneath its maximum of 6V. I chose to divide this down to ~2.93V (instead of 5V) and use 3.3V as the Vin for both the ADCMP393 and the AD7997 because the AD7997 wants to drive it's I2C at the same voltage as its Vin and I need this capped at 3.3V for my MCU.
Once I added R3 & R4 I noticed I was no longer getting reliable readings from the ADC, generally speaking my samples were directional accurate but with a wide enough variance that it concerns me and I'm now trying to resolve and improve the resolution. After researching and reading for a few days I decided to post here looking for some advice.
This document https://www.analog.com/en/analog-dialogue/articles/front-end-amp-and-rc-filter-design.html describes the design of an RC filter which I've added to the diagram and enclosed it in the red box. The document describes the method for calculating but it states the input signal frequency, amplitude and ADC conversion time need to be known. The AD7997 has a conversion time of 2uS but I do not know how to identify the input signal frequency or the amplitude and after searching and reading I've not come up with something that tells me how to define this and am looking for assistance with these.
I have placed R3 & R4 both on the IN+ of the TI LM2902 as my schematic shows and on OUT to see if doing so made any difference and I did not see any appreciable difference in my ADC readings.
Regarding Ext R1 & Ext R2 - these are external to my device and one or both of them may or may not be present on a given circuit. If they are present they will most typically be 1K, 2K or 5.6K so the range of resistance coming from them is 0 - 13.4K ohms accounting for 20% tolerances.
Greatly appreciate any guidance or feedback.
thx - sean
