ADA4945-1ACPZ-R2 datasheet(38/44 Pages) AD

38 / 44 page

ADA4945-1

Data Sheet

Rev. 0 | Page 38 of 44

IMPACT OF MISMATCHES IN THE FEEDBACK

NETWORKS

Even if the external feedback networks (RF/RG) are mismatched,

the internal common-mode feedback 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.

As well as causing a noise contribution from VOCM, ratio matching

errors in the external resistors result in a degradation of the ability

of the circuit to reject input common-mode signals, similar to a

four resistors 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 the 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 depends on whether the signal

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

input signal, as shown in Figure 103, the input impedance (RIN,dm)

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

+VS

ADA4945-1

+IN

–IN

+DIN

–DIN

VOCM

VOUT, dm

Figure 103. ADA4945-1 Configured for Balanced (Differential) Inputs

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

Figure 104, the input impedance is

RIN,SE = RG1

)

( +

β2

β1

β2

β1

where:

β1 =

β2 =

ADA4945-1

RL VOUT, dm

+VS

–VS

RG1

RG2

RF2

RF1

VOCM

RIN, SE

Figure 104. ADA4945-1 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 RG1 input resistor.

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