front-end circuit for driving adc input

[han sang jun]

Hi

The above picture is DC1762A-A schematic.
I think it's a circuit that convert single-ended to differentional signal, and I want to know why the two transformers are connected like that. Is it related to harmony or noise, or is it related to the input range?

 

[barry]

I think T1 is a common-mode choke (to reduce common-mode noise). T2 converts the single-ended signal to differential.

 

[KlausST]

Hi,

this was my first thought, too.

But then I reviewed the schematic and asked myself whether this this can work at all. For a single ended input...

and with T2 also (or much better) working as a common mode filter.

Klaus

 

[barry]

You're right. A transformer will intrinsically act to reject common-mode signals. So, I guess I don't know what's going on there.

 

[FvM]

In fact there's no problem of common mode noise for a single ended input which is grounded to the ground plane. The purpose of the two-stage balun is to improve the ADC linearity by driving it with perfect symmetrical differential signal. Main disadvantage of the circuit is the relative high high pass cut-off frequency. Typical support forum question, "how can I use it for DC input"? Answer "not at all", but high frequency linearity is considerably better than DC capable true differential amplifier. Thus it's preferred to demonstrate ADC performance in AC- only applications.