AD8117 datasheet(21/32 Pages) AD

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Preliminary Technical Data

AD8117/AD8118

Rev. PrA | Page 21 of 32

where

RG is 2.5 k , RS is the user single-ended source resistance

(such as 37.5

for a back-terminated 75

source), and

RF is

2.538 k

for the AD8117 and 5.075 k

for the AD8118.

In the case of the AD8117, this is

In the case of the AD8118, this is

When operating with a differential input, care must be taken to

keep the common-mode, or average, of the input voltages

within the linear operating range of the AD8117/AD8118

receiver. This common-mode range can extend rail-to-rail,

provided the differential signal swing is small enough to avoid

forward biasing the ESD diodes (it is safest to keep the

common-mode plus differential signal excursions within the

supply voltages of the part).

The differential output of the AD8117/AD8118 receiver is

linear for a peak of 1.4V of output voltage difference (1.4 V

peak input difference for the AD8117, and 0.7 V peak input

difference for the AD8118). Taking the output differentially,

using the two output phases, this allows 2.8 VPP of linear output

signal swing. Beyond this level, the signal path will saturate and

limit the signal swing. This is not a desired operation, as the

supply current will increase and the signal path will be slow to

recover from clipping. The absolute maximum allowed

differential input signal is limited by long-term reliability of the

input stage. The limits in the Absolute Maximum Ratings

section of the datasheet should be observed in order to avoid

degrading device performance permanently.

Ω

OPn

AD8117

ONn

RCVR

IPn

INn

Figure 23. Example of Input Driven Differentially

AC-Coupling

It is possible to AC-couple the inputs of the AD8117/AD8118

receiver. This is simplified in that bias current does not need to

be supplied externally. A capacitor in series with the inputs to

the AD8117/AD8118 will create a high-pass filter with the input

impedance of the device. This capacitor will need to be sized

such that the corner frequency is low enough for frequencies of

interest.

Single-ended Input

The AD8117/AD8118 input receiver can also be driven single-

ended (unbalanced). From the standpoint of the receiver, there

is very little difference between signals applied positive and

negative in two phases to the input pair, versus a signal applied

to one input only with the other input held at a constant

potential. One small difference is that the common-mode

between the input pins will be changing if only one input is

moving, and there is a very small common-mode to differential

conversion gain in the receiver that will add an additional gain

error to the output (see the common-mode rejection ratio

specifications for the input stage). For low frequencies, this

gain error is negligible. The common-mode rejection ratio

degrades with increasing frequency.

When operating the AD8117/AD8118 receiver single-endedly,

the observed input resistance at each input pin is higher than in

the differential input case, due to a fraction of the receiver

internal output voltage appearing as a common-mode signal on

its input terminals, bootstrapping the voltage on the input

resistance. This single-ended input resistance can be calculated

by the formula

where

RG is 2.5 k , RS is the user single-ended source resistance

(such as 37.5

for a back-terminated 75

source), and

RF is

2.538 k

for the AD8117 and 5.075 k

for the AD8118.

In most cases, a single-ended input signal will be referred to

mid-supply, typically ground. In this case, the undriven

differential input could be connected to ground. For best

dynamic performance and lowest offset voltage, this unused

input should be terminated with an impedance matching the

driven input, instead of being directly shorted to ground. Due

to the differential feedback of the receiver, there is high-

frequency signal current in the undriven input and it should be

treated as a signal line in the board design.

RIN =

RG + RS

1 –

2 × (

RG + RS + RF)

+ RF

Gdm =

5.075 k

2.5 k

Gdm =

2.538 k

2.5 k

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