ADA4937-2YCPZ-R2 datasheet(20/28 Pages) AD

20 / 28 page

ADA4937-1/ADA4937-2

Data Sheet

Rev. F | Page 20 of 28

For the source termination to be 50 Ω, the termination

resistor (RT) is calculated using RT||RIN = 50 Ω, which

makes RT equal to 61.9 Ω.

ADA4937

+VS

–VS

50Ω

200Ω

VOCM

50Ω

61.9Ω

Figure 57. Adding Termination Resistor RT

To compensate for the imbalance of the gain resistors,

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

inverting Input Gain Resistor RG. RTS is equal to the

Thevenin equivalent of the Source Resistance RS||RT.

50Ω

61.9Ω

RTH

27.4Ω

VTH

1.1V

Figure 58. Calculating Thevenin Equivalent

RTS = RTH = RS||RT = 27.4 Ω. Note that VTH is not equal to VS/2,

which is the case if the termination is not affected by the

amplifier circuit.

ADA4937

0.97V

+VS

–VS

RTH

27.4Ω

200Ω

VOCM

VTH

1.1V

RTS

27.4Ω

Figure 59. Balancing Gain Resistor RG

The feedback resistor is calculated to adjust the output

voltage.

To make the output voltage VOUT = 1 V, RF must be

calculated using the following formula:

207

)

200

(

)

(







































OUT

To make VO = VS = 2 V to recover the loss due to the input

termination, RF must be

414

)

200

(

)

(







































OUT

ADA4937

+VS

–VS

50Ω

200Ω

VOCM

61.9Ω

RTS

27.4Ω

Figure 60. Complete Single-Ended-to-Differential System

INPUT COMMON-MODE VOLTAGE RANGE IN

SINGLE-SUPPLY APPLICATIONS

The ADA4937-1/ADA4937-2 are 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, and 0.3 V to 1.2 V with a 3.3 V supply. 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

The VOCM pin of the ADA4937-1/ADA4937-2 is internally biased

at a voltage approximately equal to the midsupply point, [(+VS) +

(−VS)]/2. 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. The output

common-mode offset listed in Table 2 and Table 4 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

level (CML) 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

ADA4937-1/ADA4937-2 devices share one reference output, it is

recommended that a buffer be used.

Table 10 and Table 11 list several common gain settings, asso-

ciated resistor values, input impedances, and output noise density

values for both balanced and unbalanced input configurations.

POWER-DOWN OPERATION

The ADA4937-1/ADA4937-2 power-down pin features an

internal 25 kΩ pull-up resistor to the positive supply (+VS).

This ensures that, with the power-down pin left unconnected

(floating), the ADA4937-1/ADA4937-2 turn on. Applying a

voltage of ≤1 V turns the ADA4937-1/ADA4937-2 off.

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