Differential line receiver input stage

AI Thread Summary
The discussion centers on the AM26LS32 differential line receiver, specifically the input stage circuit diagram and its components. The input stage resembles a push-pull output stage, with 100K and 20K resistors serving to bias the A and B inputs above and below mid-rail, respectively. A user reports observing a 300mV DC offset with a 5V square wave input and a small DC current of approximately 40uA flowing from the input pins, raising questions about potential bias changes. Participants suggest verifying resistor tolerances and adjusting values in the schematic for accuracy. The conversation highlights the complexities of understanding circuit behavior in differential receivers.
likephysics
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I was going thu AM26LS32 diff line receiver. The data sheet has a ckt diagram for the input stage. But I don't understand it.
I have attached the input stage from datasheet(page3)
The input stage actually looks like a push pull output stage.
What is the function of the 100K resistors and the 20K resistor.

http://images.elektroda.net/95_1307096582.jpg
 
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likephysics said:
I was going thu AM26LS32 diff line receiver. The data sheet has a ckt diagram for the input stage. But I don't understand it.
I have attached the input stage from datasheet(page3)
The input stage actually looks like a push pull output stage.
What is the function of the 100K resistors and the 20K resistor.

http://images.elektroda.net/95_1307096582.jpg

That's just the input bias portion of the receiver inputs. It looks like they are biasing the A and B inputs to slightly above and below (respectively) mid-rail.
 
I hooked up a circuit with a voltage divider at the input.
The differential input signal at the input of the diff receiver has a DC offset of 300mV when input is 5V square wave.
A small dc current (~40uA) is actually flowing out of the diff receiver input pins.
It doesn't make sense.
Is the bias being changed some how?
See attached pdf for ckt.
 

Attachments

Anyone?
 
likephysics said:
I hooked up a circuit with a voltage divider at the input.
The differential input signal at the input of the diff receiver has a DC offset of 300mV when input is 5V square wave.
A small dc current (~40uA) is actually flowing out of the diff receiver input pins.
It doesn't make sense.
Is the bias being changed some how?
See attached pdf for ckt.

Sorry, what is the relationship of the simulation schematic in this PDF to the original input schematic in your first post?
 
berkeman said:
Sorry, what is the relationship of the simulation schematic in this PDF to the original input schematic in your first post?
The PDF is just to give you an idea of the ckt.
 
What is the tolerance of your resistors? Can you measure them as accurately as possible and change the values in your schematic accordingly?
 
skeptic2 said:
What is the tolerance of your resistors? Can you measure them as accurately as possible and change the values in your schematic accordingly?

Sure, that wouldn't be a problem.

The tolerance is 1%.
 

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