AD8139ACP-R2 datasheet(20/24 Pages) AD

20 / 24 page

AD8139

Rev. B | Page 20 of 24

For proper operation, the voltages at VAN and VAP must stay

within their respective linear ranges.

Calculating Input Impedance

The input impedance of the circuit in Figure 59 depends on

whether the amplifier is being driven by a single-ended or a

differential signal source. For balanced differential input

signals, the differential input impedance (RIN, dm) is simply

RIN, dm = 2RG

(18)

For a single-ended signal (for example, when VIN is grounded

and the input signal drives VIP), the input impedance becomes

)

(

−

(19)

The input impedance of a conventional inverting op amp

configuration is simply RG, but it is higher in Equation 19

because a fraction of the differential output voltage appears at

the summing junctions, VAN and VAP. This voltage partially

bootstraps the voltage across the input resistor RG, leading to

the increased input resistance.

Input Common-Mode Swing Considerations

In some single-ended-to-differential applications, when using a

single-supply voltage, attention must be paid to the swing of the

input common-mode voltage, VACM.

Consider the case in Figure 61, where VIN is 5 V p-p swinging

about a baseline at ground, and VREF is connected to ground.

The circuit has a differential gain of 1.6 and β = 0.38. VICM has

an amplitude of 2.5 V p-p and is swinging about ground. Using

the results in Equation 16, the common-mode voltage at the

inputs of the AD8139, VACM, is a 1.5 V p-p signal swinging

about a baseline of 0.95 V. The maximum negative excursion

of VACM in this case is 0.2 V, which exceeds the lower input

common-mode voltage limit.

DGND AGND REFGND REF REFBUFIN PDBUF

ADR431

2.5V

REFERENCE

47µF

AD7674

IN+

IN–

AVDD

DVDD

AD8139

–

VREF

2.5V

VOCM

200Ω

324Ω

20Ω

15Ω

VIN

2.7nF

0.1µF

+1.7V

+0.95V

VACM

WITH VREF = 0

+0.2V

+2.5V

GND

–2.5V

Figure 61. AD8139 Driving AD7674, 18-Bit, 800 kSPS ADC

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