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ADG5423 external resistors selection

Question asked by AlexKu on Aug 4, 2014
Latest reply on Aug 6, 2014 by seanbrown

Hello,

 

I'm thinking of building up the circuit shown bellow and will appreciate your opinion on the usage of the ADG5423.

The purpose of the circuit is to close the contacts of an opto-coupler (AQV252GAX) upon appearance of voltage at the trigger input (connected to R1).

The problem is that the trigger voltage source is very sensitive. So:

1. The load that the trigger source "sees" should be large (hundreds of Kohm).

2. This load must be disconnected after the trigger operation (i.e. a latching trigger).

 

The circuit should operate as follows:

Once voltage is applied to the trigger input, the trigger current flows through the NC (S2,D2) contacts and generates a voltage drop on R2 which is connected to IN1. IN1 in now at logic '1' and closes the NO contacts (S1,D1). This causes current to flow through the photo-diode (closing the opto-coupler contacts) and generates a logic '1' at IN2 and IN1. This causes the NC contacts to open (thus disconnecting the trigger source from the circuit) and keeps the NO contacts closed (thus forming the required latching operation).

 

My main concern is with the values of resistors R1&R2. If they are too small, I will overload my trigger voltage source. If they are too big, the transistors might not switch into conduction. Since the datasheet doesn't provide information regarding the internal transistors, I am not sure of the values that I should choose for R1 and R2.

Will the ADG5423 work as expected with R1=R2=400K?

 

Thank you in advance,

Alex

 

ADG5423_circuit.jpg

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