what will be the input impedance of PGA of AD7124-4 for gain=64
basically i want to introduce 230v AC protection at the input stage
For AD7124, With Gain > 1, the input buffers are automatically enabled. Thus, it can handle any significant source impedances which allows external filtering necessary in your application. The differential input current is 0.2nA for low power mode, 0.4nA for mid power mode and 1.5nA for full power mode. These values can be found in datasheet specification section. So, you can calculate the input impedance using the formula: Impedance = Input Voltage/Input Current. So for example, if the input voltage is 2.5V, then the impedance is 2.5V/1.5nA = 1.67Gohm at full power mode. Note that this is a worst case scenario.
thanks for your reply
i will be interfacing a k type thermocouple whose output will be in the range of 0.4mV to 48mV.
so the best impedance offered by PGA will be 133.33Kohm to 16MOhm
if i am planning for 230VAC protection at the input which will require around 50K ressistor at the input. this high value will hamper my thermocouple signal.
so now i feel i cannot provide this protection with ad7124..
if you can suggest me some other option then kindly let me know
We usually suggest to add external Schottky diodes and current limiting resistor to the input that is likely to have an overvoltage or exceed the absolute maximum ratings. Connecting external schottky diodes from each analog input to AVDD and GND ensures that the external diode turns on before the ADC’s internal diodes so that the overvoltage is directed away from the ADC. The schottky diode must have a turn-on voltage of 300mV. It will clamp applied voltages to within ~0.3V of the supply so the majority of the current will be diverted via the external diodes (which can carry higher current) and not through the internal ESD protection diodes.
thanks again for your reply
the issue here is that i need to limit the continuous current to around 5mA. which will need a 65k resistor (=230*1.414/0.005).
this 65k resistor will definately cause ratiometric attenuation if the input impedance of PGA of AD7124 is 133kohm only
I'll suggest you can have an external analog front end to isolate this high input impedance. Usually, a buffer amplifier with high input impedance and very low input bias current will do to minimize the offset voltage. It is necessary to take note some important considerations in designing your signal conditioning to meet your required specifications. For queries regarding amplifiers you can visit Amplifiers Community.
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