AD9877 datasheet(33/37 Pages) AD

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AD9877

Rev. B | Page 32 of 36

Figure 43 shows the schematic of a possible transformer

coupled circuit. Transformers with turn ratios (n2/n1) other than

1 can be selected to optimize the performance of a given

application. For example, selecting a transformer with a higher

impedance ratio (such as minicircuits T16 to 6T with an

impedance ratio of (z2/z1) = 16 = (n2/n1)2) effectively steps up

the signal amplitude, thus further reducing the output voltage

swing of the signal source.

AINP

AINN

AD9877

Figure 43. Transformer Coupled Input

In Figure 43, a resistor R1 is added between the analog inputs to

match the source impedance R as in the formula

R = [R1||RAIN](Z1/Z2)

ADC VOLTAGE REFERENCES

The AD9877 has two independent internal references for its

8-bit and 12-bit ADCs. Both 8-bit ADCs have a 1 V p-p input

and share one internal reference source. The 12-bit ADC,

however, is designed for 2 V p-p input voltages and provides its

own internal reference. Figure 16 shows the proper connections

of the reference pins REFT and REFB.

External references may be necessary for systems that require

high accuracy gain matching between ADCs or improvements

in temperature drift and noise characteristics. External

references REFT and REFB need to be centered at AVDD/2

with offset voltages as specified:

REFT8: AVDDIQ/2 + 0.25 V

REFB8: AVDDIQ/2 − 0.25 V

REFT12: AVDD/2 + 0.5 V

REFB12: AVDD/2 − 0.5 V

A differential level of 0.5 V between the reference pins results in

a 1 V p-p ADC input level AIN. A differential level of 1 V

between the reference pins results in a 2 V p-p ADC input level

AIN. Internal reference sources can be powered down when

external references are used (Register Address 0x002).

AD9877 Datasheet(PDF) 33 Page - Analog Devices