ADA4927-1/ADA4927-2

Data Sheet

Rev. B | Page 20 of 25

For an unbalanced, single-ended input signal (see Figure 49),

the input impedance is

(

) 























+

×

−

=

F

G

F

G

SE

IN

R

R

R

R

R

2

1

,

ADA4927

RL VOUT, dm

+VS

–VS

RG

RG

RF

RF

VOCM

RIN, SE

Figure 49. The ADA4927 with Unbalanced (Single-Ended) Input

The input impedance of the circuit is effectively higher than it

would be for a conventional operational amplifier connected as

an inverter because a fraction of the differential output voltage

appears at the inputs as a common-mode signal, partially boot-

strapping the voltage across the input resistor RG. 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 RF and RG in the lower loop. 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 RG in the upper loop and partially bootstrapping RG.

Terminating a Single-Ended Input

This section deals with how to properly terminate a single-

ended input to the ADA4927 with a gain of 1, RF = 348 Ω, and

RG = 348 Ω. An example using an input source with a terminated

output voltage of 1 V p-p and a source resistance of 50 Ω illustrates

the four simple steps that must be followed. Note that, because

the terminated output voltage of the source is 1 V p-p, the open

circuit output voltage of the source is 2 V p-p. The source shown

in Figure 50 indicates this open-circuit voltage.

1. The input impedance must be calculated using the following

formula:

Ω

464

)

348

348

(

2

348

1

348

)

(

2

1

=

























+

×

−

=

























+

×

−

=

F

G

F

G

IN

R

R

R

R

R

RS

50Ω

VS

2V p-p

RIN

464Ω

ADA4927

RL VOUT, dm

+VS

–VS

RG

348Ω

RG

348Ω

RF

348Ω

RF

348Ω

VOCM

Figure 50. Calculating Single-Ended Input Impedance RIN

2. To match the 50 Ω source resistance, the termination

resistor, RT, is calculated using RT||464 Ω = 50 Ω. The

closest standard 1% value for RT is 56.2 Ω.

ADA4927

RL

VOUT, dm

+VS

–VS

RS

50Ω

RG

348Ω

RG

348Ω

RF

348Ω

RF

348Ω

VOCM

VS

2V p-p

RIN

50Ω

RT

56.2

Ω

Figure 51. Adding Termination Resistor RT

3.

It can be seen from Figure 51 that the effective RG in the

upper feedback loop is now greater than the RG in the

lower loop due to the addition of the termination resistors.

To compensate for the imbalance of the gain resistors,

a correction resistor (RTS) is added in series with RG in the

lower loop. RTS is equal to the Thevenin equivalent of the

source resistance RS and the termination resistance RT and

is equal to RS||RT.

RS

50Ω

VS

2V p-p

RT

56.2

Ω

RTH

26.5Ω

VTH

1.06V p-p

Figure 52. Calculating the Thevenin Equivalent