AD8138
CALCULATING AN APPLICATION CIRCUIT’S INPUT
IMPEDANCE
When using the AD8138 in gain configurations where
RF
RG
The effective input impedance of a circuit such as the one in
Figure 42, at +DIN and –DIN, depends on whether the amplifier is
being driven by a single-ended or differential signal source. For
of one feedback network is unequal to
balanced differential input signals, the input impedance (RIN, dm
)
RF
RG
between the inputs (+DIN and −DIN) is simply
R
IN, dm =2 × RG
of the other network, there is a differential output noise due to
input-referred voltage in the VOCM circuitry. The output noise is
defined in terms of the following feedback terms (refer to
Figure 42):
In the case of a single-ended input signal (for example if −DIN is
grounded and the input signal is applied to +DIN), the input
impedance becomes
⎛
⎜
⎞
⎟
RG
β1 =
RG
RF
RG + RF
⎜
⎜
⎟
⎟
RF + RG
RIN, dm
=
1−
⎜
⎟
2×
(
)
⎝
⎠
for −OUT to +IN loop, and
RG
β2 =
The circuit’s input impedance is effectively higher than it would
be for a conventional op amp connected as an inverter because
a fraction of the differential output voltage appears at the inputs
as a common-mode signal, partially bootstrapping the voltage
across the input resistor RG.
RF + RG
for +OUT to −IN loop. With these defined,
⎡
⎢
⎤
⎥
β1 − β2
β + β
VnOUT, dm = 2VnIN,V
OCM
INPUT COMMON-MODE VOLTAGE RANGE IN
SINGLE-SUPPLY APPLICATIONS
⎢
⎣
⎥
⎦
1
2
where VnOUT, dm is the output differential noise, and V
is
nIN,VCOM
The AD8138 is optimized for level-shifting, ground-referenced
input signals. For a single-ended input, this would imply, for
example, that the voltage at −DIN in Figure 42 would be 0 V
when the amplifier’s negative power supply voltage (at V−) is
also set to 0 V.
the input-referred voltage noise in VOCM
.
THE IMPACT OF MISMATCHES IN THE FEEDBACK
NETWORKS
As previously mentioned, even if the external feedback
networks (RF/RG) are mismatched, the internal common-mode
feedback loop still forces the outputs to remain balanced. The
amplitudes of the signals at each output remains equal and 180°
out of phase. The input-to-output differential-mode gain varies
proportionately to the feedback mismatch, but the output
balance is unaffected.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The AD8138’s VOCM pin 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.
Ratio matching errors in the external resistors result in a
degradation of the circuit’s ability to reject input common-
mode signals, much the same as for a four-resistor difference
amplifier made from a conventional op amp.
In cases where more accurate control of the output common-
mode level is required, it is recommended that an external
source, or resistor divider (made up of 10 kΩ resistors), be used.
The output common-mode offset listed in the Specifications
section assumes the VOCM input is driven by a low impedance
voltage source.
In addition, if the dc levels of the input and output common-
mode voltages are different, matching errors result in a small
differential-mode output offset voltage. For the G = 1 case, with
a ground referenced input signal and the output common-mode
level set for 2.5 V, an output offset of as much as 25 mV (1% of
the difference in common-mode levels) can result if 1% tolerance
resistors are used. Resistors of 1% tolerance result in a worst-
case input CMRR of about 40 dB, worst-case differential mode
output offset of 25 mV due to 2.5 V level-shift, and no significant
degradation in output balance error.
DRIVING A CAPACITIVE LOAD
A purely capacitive load can react with the pin and bondwire
inductance of the AD8138, resulting in high frequency ringing
in the pulse response. One way to minimize this effect is to
place a small capacitor across each of the feedback resistors. The
added capacitance should be small to avoid destabilizing the
amplifier. An alternative technique is to place a small resistor in
series with the amplifier’s outputs, as shown in Figure 40.
Rev. F | Page 18 of 24
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