AD8138ARZ-R7 Analog Devices Inc, AD8138ARZ-R7 Datasheet - Page 20

IC AMP DIFF LDIST LP 95MA 8SOIC

AD8138ARZ-R7

Manufacturer Part Number
AD8138ARZ-R7
Description
IC AMP DIFF LDIST LP 95MA 8SOIC
Manufacturer
Analog Devices Inc
Type
ADC Driverr
Datasheet

Specifications of AD8138ARZ-R7

Design Resources
DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7352 (CN0040) DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7356 (CN0041) DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7357 (CN0061)
Applications
Data Acquisition
Mounting Type
Surface Mount
Package / Case
8-SOIC (3.9mm Width)
Number Of Channels
1
Number Of Elements
1
Power Supply Requirement
Single/Dual
Common Mode Rejection Ratio
70dB
Voltage Gain Db
0.028dB
Input Resistance
6@5VMohm
Input Offset Voltage
2.5@5VmV
Input Bias Current
7@5VnA
Single Supply Voltage (typ)
3/5/9V
Dual Supply Voltage (typ)
±3/±5V
Power Supply Rejection Ratio
70dB
Power Dissipation
550mW
Rail/rail I/o Type
No
Single Supply Voltage (min)
2.7V
Single Supply Voltage (max)
11V
Dual Supply Voltage (min)
±1.4V
Dual Supply Voltage (max)
±5.5V
Operating Temp Range
-40C to 85C
Operating Temperature Classification
Industrial
Mounting
Surface Mount
Pin Count
8
Package Type
SOIC N
Lead Free Status / RoHS Status
Lead free / RoHS Compliant

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Quantity
Price
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Quantity:
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Part Number:
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AD8138
BALANCED TRANSFORMER DRIVER
Transformers are among the oldest devices used to perform a
single-ended-to-differential conversion (and vice versa). Trans-
formers can also perform the additional functions of galvanic
isolation, step-up or step-down of voltages, and impedance
transformation. For these reasons, transformers always find
uses in certain applications.
However, when driving the transformer in a single-ended
manner, there is an imbalance at the output due to the parasitics
inherent in the transformer. The primary (or driven) side of the
transformer has one side at dc potential (usually ground), while
the other side is driven. This can cause problems in systems that
require good balance of the transformer’s differential output
signals.
If the interwinding capacitance (C
uniformly distributed, a signal from the driving source couples
to the secondary output terminal that is closest to the primary’s
driven side. On the other hand, no signal is coupled to the
opposite terminal of the secondary because its nearest primary
terminal is not driven (see Figure 43). The exact amount of this
imbalance depends on the particular parasitics of the trans-
former, but is mostly a problem at higher frequencies.
The balance of a differential circuit can be measured by
connecting an equal-valued resistive voltage divider across the
differential outputs and then measuring the center point of the
circuit with respect to ground. Since the two differential outputs
are supposed to be of equal amplitude, but 180° opposite phase,
there should be no signal present for perfectly balanced outputs.
The circuit in Figure 43 shows a Mini-Circuits® T1-6T
transformer connected with its primary driven single-endedly
and the secondary connected with a precision voltage divider
across its terminals. The voltage divider is made up of two
500 Ω, 0.005% precision resistors. The voltage V
also equal to the ac common-mode voltage, is a measure of how
closely the outputs are balanced.
Figure 45 compares the transformer being driven single-
endedly by a signal generator and being driven differentially
using an AD8138. The top signal trace of Figure 45 shows the
balance of the single-ended configuration, while the bottom
shows the differentially driven balance response. The 100 MHz
balance is 35 dB better when using the AD8138.
STRAY
) is assumed to be
UNBAL
, which is
Rev. F | Page 20 of 24
The well-balanced outputs of the AD8138 provide a drive signal
to each of the transformer’s primary inputs that are of equal
amplitude and 180° out of phase. Therefore, depending on how
the polarity of the secondary is connected, the signals that
conduct across the interwinding capacitance either both assist
the transformer’s secondary signal equally, or both buck the
secondary signals. In either case, the parasitic effect is
symmetrical and provides a well-balanced transformer output
(see Figure 45).
Figure 43. Transformer Single-Ended-to-Differential Converter Is Inherently
Figure 45. Output Balance Error for Circuits of Figure 43 and Figure 44
–100
–20
–40
–60
–80
499Ω
0
0.3
Figure 44. AD8138 Forms a Balanced Transformer Driver
52.3Ω
499Ω
PRIMARY
ON THIS SIDE VIA C
NO SIGNAL IS COUPLED
SIGNAL IS COUPLED
+IN
–IN
499Ω
499Ω
V
WITH SINGLE-ENDED DRIVE
ON THIS SIDE
UNBAL
1
AD8138
C
C
STRAY
STRAY
, FOR TRANSFORMER
FREQUENCY (MHz)
Imbalanced
OUT–
OUT+
49.9Ω
49.9Ω
V
UNBAL
STRAY
V
UNBAL
10
C
C
500Ω
0.005%
500Ω
0.005%
STRAY
STRAY
, DIFFERENTIAL DRIVE
SECONDARY V
V
UNBAL
100
500Ω
0.005%
500Ω
0.005%
DIFF
V
500
DIFF

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