AD8117_07 AD [Analog Devices], AD8117_07 Datasheet - Page 28

• Manufacturer Part Number: AD8117_07
• Manufacturer: AD [Analog Devices]

AD8117/AD8118
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). See the Specifications section for guaranteed input
range.
The differential output of the AD8117/AD8118 receiver is
linear for a peak of 1.4 V 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 V p-p of linear
output signal swing. Beyond this level, the signal path can
saturate and will limit the signal swing. This is not a desired
operation, as the supply current increases and the signal path
will be slow to recover from clipping. The absolute maximum
allowed differential input signal is limited by the long-term
reliability of the input stage. The limits in the Absolute
Maximum Ratings section should be observed in order to avoid
degrading device performance permanently.
Single-Ended Input
The AD8117/AD8118 input receivers can be driven single-
endedly (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, vs. 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 is changing if only one input is moving,
and there is a very small common-mode to differential
conversion gain in the receiver that adds an additional gain
error to the output (see the common-mode rejection ratio for
the input stage in the Specifications section). For low
frequencies, this gain error is negligible. The common-mode
rejection ratio degrades with increasing frequency.
When operating the AD8117/AD8118 receivers single-endedly,
the observed input resistance at each input pin is lower 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 equation
50Ω
Figure 66. Example of Input Driven Differentially
50Ω
INn
IPn
RCVR
AD8117
OPn
ONn
Rev. A | Page 28 of 36
where:
R
R
a back-terminated 75 Ω source).
R
In most cases, a single-ended input signal is referred to
midsupply, typically ground. In this case, the undriven
differential input can 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.
AC Coupling of Inputs
It is possible to ac couple the inputs of the AD8117/AD8118
receiver. This is simplified because the bias current does not
need to be supplied externally. A capacitor in series with the
inputs to the AD8117/AD8118 creates a high-pass filter with
the input impedance of the device. This capacitor needs to be
sized such that the corner frequency is low enough for
frequencies of interest.
Differential Output
Benefits of Differential Operation
The AD8117/AD8118 have a fully differential switch core, with
differential outputs. The two output voltages move in opposite
polarity, with a differential feedback loop maintaining a fixed
output stage differential gain of +1 (the different overall signal
path gains between the AD8117 and AD8118 are set in the
input stage for best signal-to-noise ratio). This differential
output stage provides a benefit of crosstalk-canceling due to
parasitic coupling from one output to another, being equal and
out of phase. Additionally, if the output of the device is utilized
in a differential design, noise, crosstalk, and offset voltages
generated on-chip that are coupled equally into both outputs are
cancelled by the common-mode rejection ratio of the next
device in the signal chain. By utilizing the AD8117/AD8118
outputs in a differential application, the best possible noise and
offset specifications can be realized.
G
S
F
is the user single-ended source resistance (such as 37.5 Ω for
= 2.538 kΩ for the AD8117 and 5.075 kΩ for the AD8118.
= 2.5 kΩ.
R
IN
75Ω
=
1
Figure 67. Example of Input Driven Single-Ended
−
2
75Ω
(OR 37.5Ω)
×
(
R
R
G
INn
G
IPn
+
+
R
R
F
R
S
S
+
RCVR
R
F
AD8117
)
OPn
ONn