Preliminary Technical Data

ADA4938-1

Rev. PrD | Page 11 of 14

Similar to the case of a conventional op amp, the output noise

voltage densities can be estimated by multiplying the input-

referred terms at +IN and −IN by the appropriate output factor,

where:

()

2

1

N

β

β

G

+

=

2

is the circuit noise gain.

G1

F1

G1

1

R

R

R

β

+

=

and

G2

F2

G2

2

R

R

R

β

+

=

are the feedback factors.

becomes

G

F

N

R

R

β

G

+

=

=

1

1

square of the individual output noise terms.

∑

=

=

8

1

i

2

nOi

nOD

v

v

THE IMPACT OF MISMATCHES IN THE FEEDBACK

NETWORKS

As previously mentioned, even if the external feedback networks

loop still forces the outputs to remain balanced. The amplitudes

of the signals at each output remain equal and 180° out of phase.

The input-to-output, differential mode gain varies proportionately

to the feedback mismatch, but the output balance is unaffected.

matching errors in the external resistors result in a degradation

of the ability of the circuit to reject input common-mode signals,

much the same as for a four-resistor difference amplifier made

from a conventional op amp.

In addition, if the dc levels of the input and output common-

mode voltages are different, matching errors result in a small

differential-mode output offset voltage. When G = 1, with a

ground referenced input signal and the output common-mode

level set to 2.5 V, an output offset of as much as 25 mV (1% of

the difference in common-mode levels) can result if 1% tolerance

resistors are used. Resistors of 1% tolerance result in a worst-

case input CMRR of about 40 dB, a worst-case differential-

mode output offset of 25 mV due to 2.5 V level-shift, and no

significant degradation in output balance error.

CALCULATING THE INPUT IMPEDANCE OF AN

APPLICATION CIRCUIT

The effective input impedance of a circuit depends on whether

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

signal source. For balanced differential input signals, as shown

Figure 6. ADA4938-1 Configured for Balanced (Differential) Inputs

For an unbalanced, single-ended input signal (see Figure 7), the

input impedance is

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎝

⎛

+

×

−

=

F

G

F

G

cm

IN

R

R

R

R

R

2

1

,

Figure 7. ADA4938-1 Configured for Unbalanced (Single-Ended) Input

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

INPUT COMMON-MODE VOLTAGE RANGE IN

SINGLE-SUPPLY APPLICATIONS

The ADA4938-1 is optimized for level-shifting, ground-referenced

input signals. As such, the center of the input common-mode

range is shifted approximately 1 V down from midsupply. For

5 V single-supply operation, the input common-mode range at

the summing nodes of the amplifier is 0.3 V to 3.0 V. To avoid

clipping at the outputs, the voltage swing at the +IN and –IN

terminals must be confined to these ranges.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

approximately equal to the midsupply point (average value of

the voltages on V+ and V−). Relying on this internal bias results

in an output common-mode voltage that is within about 100 mV of

the expected value.

In cases where more accurate control of the output common-

mode level is required, it is recommended that an external

source, or resistor divider (10 kΩ or greater resistors), be used.

level (CML) output of an ADC. However, care must be taken to