AD8042 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8042 Datasheet PDF : 16 Pages
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AD8042
Single-Ended-to-Differential Driver
Using a cross-coupled, single-ended-to-differential converter
(SEDC), the AD8042 makes a good general-purpose differential
line driver. This SEDC can be used for applications such as
driving Category-5 (CAT-5) twisted pair wires. Figure 39 shows
a configuration for a circuit that performs this function that can
be used for video transmission over a differential pair or various
data communication purposes.
+5V
0.1µF 10µF
VIN
RIN
1kΩ 3
8
RF
1kΩ
1
60.4Ω
49.9Ω 2 AMP1
RA
1kΩ
50m
AD8042
RB
1kΩ
RB
1kΩ
121Ω VOUT
100Ω
RA
6 1kΩ
7
5 AMP2
4
0.1µF
–5V
60.4Ω
10µF
Figure 39. Single-Ended-to-Differential Twisted Pair Line Driver
Each of the op amps of the AD8042 is configured as a unity gain
follower by the feedback resistors (RA). Each op amp output also
drives the other as a unity gain inverter via RB,B creating a totally
symmetrical circuit.
If the noninverting input of AMP2 is grounded and a small
positive signal is applied to the noninverting input of AMP1,
the output of AMP1 is driven to saturation in the positive
direction and the input of AMP2 is driven to saturation in the
negative direction. This is similar to the way a conventional op
amp behaves without any feedback.
If a resistor (RF) is connected from the output of AMP2 to the
noninverting input of AMP1, negative feedback is provided, which
closes the loop. An input resistor (RIN) makes the circuit look
like a conventional inverting op amp configuration with
differential outputs.
The gain of this circuit from input to either output is ±RF/RIN, or
the single-ended-to-differential gain is 2 × RF/RIN. This gives the
circuit the advantage of being able to adjust its gain by changing
a single resistor.
The cable has a characteristic impedance of about 120 Ω. Each
driver output is back terminated with a pair of 60.4 Ω resistors
to make the source look like 120 Ω. The receive end is terminated
with 121 Ω, and the signal is measured differentially with a pair
of scope probes. One channel on the oscilloscope is inverted
and then the signals are added.
Figure 40 shows the results of the circuit in Figure 39 driving
50 meters of CAT-5 cable.
1V
200mV
50ns
100
VIN 90
VOUT
10
0%
200mV
Figure 40. Differential Driver Frequency Response
Single-Supply Differential A/D Driver
The single-ended-to-differential converter circuit is also useful
as a differential driver for video speed, single-ended, differential
input ADCs. Figure 41 is a schematic that shows such a circuit
differentially driving an AD9220, a 12-bit, 10 MSPS ADC.
+5V
+5V
0.1µF
0.1µF 1kΩ
VIN
3
1kΩ
8
1
2
+5V
+5V
+5V
1kΩ
0.1µF
0.1µF
0.1µF
AD8042 1kΩ
1kΩ
28
DVDD
15
AVDD
26
AVDD
+5V
2.49kΩ
2.49kΩ
6
5
0.1µF
1kΩ
VINA
7
VINB
4
0.1µF
0.1µF
CAPT
10/16 0.1µF
CAPB
18
VREF
17
SENSE
22
0.1µF
CML
CLOCK
1
CLK
AD9220
OTR 14
BIT 1 13
BIT 2 12
BIT 3 11
BIT 4 10
BIT 5 9
BIT 6 8
7
BIT 7
6
BIT 8
5
BIT 9
4
BIT 10
3
BIT 11
2
BIT 12
REFCOM DVSS AVSS AVSS
19
27 25 16
Figure 41. AD8042 Differential Driver for the AD9220 12-Bit, 10 MSPS ADC
Rev. E | Page 14 of 16
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