I have few questions from this CN0325 circuit. As far as I understand the (U23) ADG1204 mux is used to bias the input - to 2.5v and this is for the thermocouple connection? Why is the AD7795 internal biasing not used?
Second one is that I understand the purpose of D5 and D7 diodes, but why is there also diodes D1 and D2?
Is there any advantages using external 5v regulator ADP1720 instead of ADuM3471's internal 5v regulator?
Very appreciated for your questions. pls find my comments on your question sequently.
Yes, actually we provided two ways to clamp the differential input common voltage to 2.5V, one way is to use internal Vbias function as you mentioned, another way is to use resistor divider to limit the common voltage in front of circuit, I think both of them are work for TC.
D1,D2 intend to protect Iout1,Iout2 pin during over voltage input, but if considering D34,D35 in series of Iout1 and Iout2, strictly speaking we can remove D1,D2 with low risk for normal function, but still proposed if considering in noisy environment.
Actually you may found we leave two option for 5V selection on board, both ADP1720 and ADuM3471's interal 5V regulator, you can switch them by jumpers. While ADP1720 can bring more clean and stable(low ripple) compare to ADuM3471's internal 5V. And also, ADuM3471's internal 5V can only output limited current compare to ADP1720.
I'm working on the upgrade of one of our product, the current product is based on multiplexer and AD7793, REF193, and we need to add new signal input +/-5, +/-10V based on current product.
I have a few question about CN0325 after reviewing the "EVAL-CN0325-SDPZ-Schematic-Rev0.pdf",
1) What's the function of R99 (10mega ohm) used in Schematic "UIM WITH 6TB" or R130 (10mega ohm) used in Schematic "UIM WITH 4TB".
2) Can I use ADG508F(with fault protect) as multiplexer for CN0325 instread of using the overvoltage protection diode D20,D22 in Schematic "UIM WITH 6TB" or D5,D7 in Schematic "UIM WITH 4TB".
3) For CN0325, when multiplexer selected to be thermocouple/millivoltage channel, can ADC be protected from an overvoltage e.g. 30V input ?
Thanks and Regards
In answer to your questions above;
1) R130, R109 and R99 are optional, they ensure the inputs will not float when open circuit (They should stay at ~0V differential).
2) Using devices such as the ADG508F (can tolerate -40V to 55V) would eliminate the need for D20 and D22 for overvoltage protection. That said I would then recommend an external bi-directional TVS to protect against high voltage transients such as the IEC6000-4-x tests. The ideal if for the TVS to have a 'maximum reverse voltage' of <40V (based on the switch tolerating -40V to 55V). In this regard some of the newer fault protected devices such as the ADG5404F and ADG5412F may be of benefit as they can tolerate +55V and -55V, so the maximum reverse voltage could be as high as 55V making TVS selection much easier. The ADG5404F and ADG5412F also offer a fault pin which will alert the user when there is an overvoltage condition on the input.
3) If the thermocouple channel were selected and 30V was applied to the input then this would be clamped to 15V by D5/D7, which is still to high for the ADC... So in reality the current will flow through the internal up/down diodes of the AD7795 ADC which will clamp it to 5V. Lets assume all 30V were dropped across the 2.2kohm resistor, then that would be I = V/R = 30V/2.2kohm = 13.6mA. The AD7795 is specified for an absolute max AIN current of 10mA, so this is a bit above this rating. The simplest option to improve this is to add additional resistance between the multiplexer and AIN2+ and AIN2-. An additional 1kohm would limit the current to I = V/R = 30V/(2.2k+1k) = 9.4mA which is within the AD7795's absolute maximum ratings.