ADA4938-2ACPZ-R2 datasheet(21/26 Pages) AD

21 / 26 page

Data Sheet

ADA4938-1/ADA4938-2

Rev. B | Page 21 of 26

TERMINATING A SINGLE-ENDED INPUT

Using an example with an input source of 2 V, a source

resistance of 50 Ω, and an overall gain of 1 V/V, four simple

steps must be followed to terminate a single-ended input to the

ADA4938-1/ADA4938-2.

1. The input impedance is calculated using the formula



267

200)

(200

200











































ADA4938

+VS

–VS

50Ω

200Ω

VOCM

RIN

267Ω

Figure 61. Single-Ended Input Impedance

2. To provide a 50 Ω termination for the source, the Resistor RT

is calculated such that RT || RIN = 50 Ω, or RT = 61.9 Ω.

ADA4938

+VS

–VS

50Ω

200Ω

VOCM

50Ω

61.9Ω

Figure 62. Adding Termination Resistor RT

3. To compensate for the imbalance of the gain resistors, a correc-

tion 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 63. Calculating Thevenin Equivalent

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

VS/2, which would be the case if the amplifier circuit did

not affect the termination.

ADA4938

0.97V

+VS

–VS

RTH

27.4Ω

200Ω

VOCM

VTH

1.1V

RTS

27.4Ω

Figure 64. Balancing Gain Resistor RG

4. Finally, the feedback resistor is recalculated to adjust the

output voltage to the desired level.

To make the output voltage VO = 1 V, RF is calculated

using

207

1.1

27.4)

(200

)

(







































To return the overall gain to 1 V/V (VO = VS = 2 V), RF

should be

414

1.1

27.4)

(200

)

(







































ADA4938

+VS

–VS

50Ω

200Ω

VOCM

61.9Ω

RTS

27.4Ω

Figure 65. Complete Single-Ended-to-Differential System

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM pin of the ADA4938-1/ADA4938-2 is internally

biased at a 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.

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

ADA4938-1/ADA4938-2 devices share one reference output, it is

recommended that a buffer be used.

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