Data Sheet

ADA4930-1/ADA4930-2

Rev. D | Page 19 of 25

MINIMUM RG VALUE

Due to the wide bandwidth of the ADA4930-1/ADA4930-2, the

value of RG must be greater than orequal to 301 Ω at unity gain

to provide sufficient damping in the amplifier front end.In the

terminated case, RG includes the Thevenin resistance of the

source and load terminations.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM pin of the ADA4930-1/ADA4930-2 is biased at3/10 of

the total supply voltageabove−VS with an internal voltagedivider.

The input impedance of the VOCM pin is 8.4 kΩ. When relying

on the internal bias, the output common-mode voltage is within

about 100 mV of the expected value.

In cases where accuratecontrol of the output common-mode

level is required, it is recommended that an external source or

resistor divider be usedwith source resistanceless than 100 Ω.

The output common-modeoffset listed in the Specifications

section assumes that the VOCM input is driven by a low

impedance voltagesource.

It is also possible to connect the VOCM input to a common-mode

voltage (VCM) output of an ADC. However,care must be taken

to ensure that the output hassufficient drive capability. The

input impedance of the VOCM pin is approximately 10 kΩ. If

multiple ADA4930-1/ADA4930-2 devicesshareonereference

output, it is recommendedthat a buffer beused.

CALCULATING THE INPUT IMPEDANCE FOR AN

APPLICATIONCIRCUIT

The effective input impedancedepends on whether the signal

source is single-ended or differential. For a balanced differential

input signal, asshown in Figure44,theinput impedance(RIN, dm)

between the inputs (+DIN and −DIN)is RIN, dm = 2 × RG.

+VS

ADA4930

+IN

–IN

RF

RF

+DIN

–DIN

VOCM

RG

RG

VOUT, dm

Figure 44. ADA4930-1/ADA4930-2 Configuredfor Balanced(Differential)Inputs

For an unbalanced single-endedinput signal, as shown in

Figure 45, the input impedance is

RIN,SE = RG1

)

1

( +

+

β2

β1

β2

β1

where:

β1 =

F1

G1

G1

R

R

R

+

β2 =

2

2

F

G2

G

R

R

R

+

ADA4930

RL VOUT, dm

+VS

–VS

RG1

RG2

RF2

RF1

VOCM

RIN, SE

Figure 45. ADA4930-1/ADA4930-2 with Unbalanced (Single-Ended) Input

For a balanced systemwhereRG1= RG2 = RG and RF1 = RF2 = RF,

the equations simplify to

























+

−

=

+

=

=

)

2(

1

F

G

F

G

IN,SE

F

G

G

R

R

R

R

R

and

R

R

R

β2

β1

The input impedance of the circuit is effectively higher than it

would be for a conventional op amp connected as an inverter

because a fraction of the differential output voltage appearsat

the inputs as a common-modesignal, partially bootstrapping

the voltage across the input resistor RG1. The common-mode

voltageattheamplifierinput terminalscan be easily determined

by noting that the voltage at the inverting input is equal to the

noninverting output voltagedivideddown bythevoltagedivider

formed by RF2and RG2. This voltage is present at both input

terminals due to negativevoltagefeedbackand is in phase with

the input signal, thus reducing the effective voltage across RG1,

partially bootstrapping it.

Terminating a Single-Ended Input

This section describes the five stepsthat properly terminatea

single-ended input to the ADA4930-1/ADA4930-2. Assume a

systemgain of1, RF1= RF2= 301 Ω, an inputsourcewith an open-

circuit outputvoltageof2 V p-p, and asource resistanceof50 Ω.

Figure 46 shows this circuit.

1. Calculate the input impedance.

β1 = β2 = 301/602 = 0.5 and RIN = 401.333 Ω