How to build a Voltage limiter for a millivolt signal?

Thread Starter

anoopak

Joined Nov 24, 2023
36
I had built a millivolt conditioning circuit. A 0-40mV differential DC signal is fed to AD620 with a set gain of 100. The output of AD620 [0-4V] is fed to ADS1115 . Is there any method to limit the millivolt input to AD620 below 50mV so that ADS1115 will get saved when a higher input is accidentally fed to the input of AD620? Millivolt signal is generated from a Wheatstone bridge circuit [Common mode voltage ~10V].
 

ericgibbs

Joined Jan 29, 2010
18,854
hi,
You could consider reducing the supplies to say +/-5V.
The AD620 output saturates at approx Vs-1.2V, so the ADC could never be overdriven.
EEG57_ 1426.png
 

tonyStewart

Joined May 8, 2012
131
the AD620 limit is ; Differential Input Voltage 25 V. No protection needed.
the ADS111x limit is ; Analog input voltage AIN0, AIN1, AIN2, AIN3 GND – 0.3 VDD + 0.3 V
1706194189902.png

The input protection diodes are current limited
1706194235146.png

Thus using series R between AD620 (+/-15V ) to ADS111x (+5,0V) you need to limit the current by 15V-5V = 10V or 10V/10mA using >= 1k resistors or 2k with margin. This is easier than say using 2.5V +/-2.5V on the AD620 but is also possible then the AD620 is referenced to 2.5V instead of 0V.

This requires your Wheatstone bridge to be +ve and unipolar so that the AD620 output is between the limits for the ADC.
1706194910294.png

if your bridge is bipolar
, then the AD620 needs to be biased with a positive output, preferably to 2.5V using REF in with say 10k to +5 and 0V buffered by any Op Amp. (gain=+1) to drive REF.
1706195207572.png
 
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Thread Starter

anoopak

Joined Nov 24, 2023
36
hi,
You could consider reducing the supplies to say +/-5V.
The AD620 output saturates at approx Vs-1.2V, so the ADC could never be overdriven.
EView attachment 313525
I selected the value of power supply voltage based on Diamond plot tool [Analog devices] recommendations. It says that for a common mode voltage of 10V , power supply voltage of +15v and -15V is required for AD620 for its operation.
 

Thread Starter

anoopak

Joined Nov 24, 2023
36
the AD620 limit is ; Differential Input Voltage 25 V. No protection needed.
the ADS111x limit is ; Analog input voltage AIN0, AIN1, AIN2, AIN3 GND – 0.3 VDD + 0.3 V
I operate ADS1115 with a gain of one where the maximum input voltage is 4V. I fear if any spike at the AD620 input can expose ADS1115 to voltages more than 4V which can damage ADC [ADS1115 supply voltage is 5V].
 
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Thread Starter

anoopak

Joined Nov 24, 2023
36
the AD620 limit is ; Differential Input Voltage 25 V. No protection needed.
the ADS111x limit is ; Analog input voltage AIN0, AIN1, AIN2, AIN3 GND – 0.3 VDD + 0.3 V
Thus using series R between AD620 (+/-15V ) to ADS111x (+5,0V) you need to limit the current by 15V-5V = 10V or 10V/10mA using >= 1k resistors or 2k with margin.
This means I don't have to worry about damage of AD620 [Bridge excitation voltage is 24V] .
Connecting a resistor of 2K between AD620 and ADS1115 is convenient option for me. Whether there will be any drop in signal voltage due to this resistor?
 
Last edited:

ericgibbs

Joined Jan 29, 2010
18,854
V1 is not a voltage source but some unknown values of a Wheatstone bridge, but may work if they are low enough R values. We need to see the bridge datasheet link. Also STP cables ought to be used to reduce stray line noise, preferably with PE gnd connection to shield.
hi tony,
V1 is a simulation test Voltage source that sweeps from 0mV through 10mV.
Knowing the bridge data is not required for this type of simulation.
E
 

ericgibbs

Joined Jan 29, 2010
18,854
You recommends connecting a 4V Zener diode at AD620 output? Is R3 is the current limiting resistor for Zener? Can you tell the purpose of R1 & R2? [Sorry I am not from Electronics background]
Hi anoo,
R1 and R2 are there to bias the AD620 inputs to Vsupply/2, which is typical for a bridge input
So with a +15V bridge supply that's 15v/2 =7.5v.

R3 is a current limit resistor to the zener diode, when the Vout of the AD620 rises above ~4v.

E
 

tonyStewart

Joined May 8, 2012
131
This means I don't have to worry about damage of AD620 [Bridge excitation voltage is 24V] .
Connecting a resistor of 2K between AD620 and ADS1115 is convenient option for me. Whether there will be any drop in signal voltage due to this resistor?
No voltage drop with next to no input current in datasheet.

1706204346671.png
 
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