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AD8138AR
AD8138AR  

Manufacturer Part Number  AD8138AR 
Description  IC,Differential Amplifier,BIPOLAR,SOP,8PIN,PLASTIC 
Manufacturer  Analog Devices Inc 
Type  ADC Driver 
AD8138AR datasheet 

Specifications of AD8138AR  

Rohs Status  RoHS noncompliant  Design Resources  DCCoupled, SingleEndedtoDifferential Conversion Using AD8138 and AD7352 (CN0040) DCCoupled, SingleEndedtoDifferential Conversion Using AD8138 and AD7356 (CN0041) DCCoupled, SingleEndedtoDifferential Conversion Using AD8138 and AD7357 (CN0061) 
Applications  Data Acquisition  Mounting Type  Surface Mount 
Package / Case  8SOIC (3.9mm Width) 
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THEORY OF OPERATION
The AD8138 differs from conventional op amps in that it has
two outputs whose voltages move in opposite directions. Like
an op amp, it relies on high openloop gain and negative
feedback to force these outputs to the desired voltages. The
AD8138 behaves much like a standard voltage feedback op
amp and makes it easy to perform singleendedtodifferential
conversion, commonmode levelshifting, and amplification of
differential signals. Also like an op amp, the AD8138 has high
input impedance and low output impedance.
Previous differential drivers, both discrete and integrated
designs, have been based on using two independent amplifiers
and two independent feedback loops, one to control each of the
outputs. When these circuits are driven from a singleended
source, the resulting outputs are typically not well balanced.
Achieving a balanced output has typically required exceptional
matching of the amplifiers and feedback networks.
DC commonmode levelshifting has also been difficult with
previous differential drivers. Levelshifting has required the use
of a third amplifier and feedback loop to control the output
commonmode level. Sometimes the third amplifier has also
been used to attempt to correct an inherently unbalanced
circuit. Excellent performance over a wide frequency range
has proven difficult with this approach.
The AD8138 uses two feedback loops to separately control the
differential and commonmode output voltages. The differential
feedback, set with external resistors, controls only the differential
output voltage. The commonmode feedback controls only the
commonmode output voltage. This architecture makes it easy
to arbitrarily set the output commonmode level. It is forced, by
internal commonmode feedback, to be equal to the voltage
applied to the V
input, without affecting the differential
OCM
output voltage.
The AD8138 architecture results in outputs that are very highly
balanced over a wide frequency range without requiring tightly
matched external components. The commonmode feedback
loop forces the signal component of the output commonmode
voltage to be zeroed. The result is nearly perfectly balanced
differential outputs of identical amplitude and exactly
180° apart in phase.
ANALYZING AN APPLICATION CIRCUIT
The AD8138 uses high openloop gain and negative feedback to
force its differential and commonmode output voltages in such
a way as to minimize the differential and commonmode error
voltages. The differential error voltage is defined as the voltage
between the differential inputs labeled +IN and −IN in Figure 42.
For most purposes, this voltage can be assumed to be zero.
Similarly, the difference between the actual output common
mode voltage and the voltage applied to V
assumed to be zero. Starting from these two assumptions, any
application circuit can be analyzed.
SETTING THE CLOSEDLOOP GAIN
Neglecting the capacitors C
circuit in Figure 42 can be determined to be described by
V
S
R
OUT
,
dm
=
F
S
V
R
OUT
,
dm
G
This assumes the input resistors, R
on each side are equal.
ESTIMATING THE OUTPUT NOISE VOLTAGE
Similar to the case of a conventional op amp, the differential
output errors (noise and offset voltages) can be estimated by
multiplying the input referred terms, at +IN and −IN, by the
circuit noise gain. The noise gain is defined as
⎛
⎞
R
⎜
⎟
=
+
F
G
1
⎜
⎟
N
R
⎝
⎠
G
To compute the total output referred noise for the circuit of
Figure 42, consideration must also be given to the contribution
of the Resistors R
and R
. Refer to Table 8 for the estimated
F
G
output noise voltage densities at various closedloop gains.
Table 8.
R
R
Bandwidth
G
F
Gain
(Ω)
(Ω)
−3 dB
1
499
499
320 MHz
2
499
1.0 k
180 MHz
5
499
2.49 k
70 MHz
10
499
4.99 k
30 MHz
Rev. F  Page 17 of 24
AD8138
can also be
OCM
, the differentialmode gain of the
F
S
S
, and feedback resistors, R
,
G
F
Output
Output
Noise
Noise
AD8138
AD8138 +
Only
R
, R
G
F
10 nV/√Hz
11.6 nV/√Hz
15 nV/√Hz
18.2 nV/√Hz
30 nV/√Hz
37.9 nV/√Hz
55 nV/√Hz
70.8 nV/√Hz
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