ADA4930-2YCPZ-R2 datasheet(19/25 Pages) AD

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Data Sheet

ADA4930-1/ADA4930-2

Rev. C | 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 or equal 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 at 3/10 of

the total supply voltage above −VS with an internal voltage divider.

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 accurate control of the output common-mode

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

resistor divider be used with source resistance less than 100 Ω.

The output common-mode offset listed in the Specifications

section assumes that the VOCM input is driven by a low

impedance voltage source.

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 has sufficient drive capability. The

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

multiple ADA4930-1/ADA4930-2 devices share one reference

output, it is recommended that a buffer be used.

CALCULATING THE INPUT IMPEDANCE FOR AN

APPLICATION CIRCUIT

The effective input impedance depends on whether the signal

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

input signal, as shown in Figure 44, the input impedance (RIN,dm)

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

+VS

ADA4930

+IN

–IN

+DIN

–DIN

VOCM

VOUT, dm

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

For an unbalanced single-ended input signal, as shown in

Figure 45, the input impedance is

RIN,SE = RG1

)

( +

β2

β1

β2

β1

where:

β1 =

β2 =

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 system where RG1 = RG2 = RG and RF1 = RF2 = RF,

the equations simplify to













−

)

IN,SE

and

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 appears at

the inputs as a common-mode signal, partially bootstrapping

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

voltage at the amplifier input terminals can be easily determined

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

noninverting output voltage divided down by the voltage divider

formed by RF2 and RG2. This voltage is present at both input

terminals due to negative voltage feedback and 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 steps that properly terminate a

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

system gain of 1, RF1 = RF2 = 301 Ω, an input source with an open-

circuit output voltage of 2 V p-p, and a source resistance of 50 Ω.

Figure 46 shows this circuit.

1. Calculate the input impedance.

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

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