Using a cross-coupled, single-ended-to-differential converter,
the AD8042 makes a good general-purpose differential line
driver. This can be used for applications such as driving
Category-5 (CAT-5) twisted pair wire. 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.
49.9Ω 2 AMP1
Figure 39. Single-Ended-to-Differential Twisted Pair Line Driver
Each of the AD8042’s op amps 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 the two RBS, creating
a totally symmetrical circuit.
If the + input to Amp2 is grounded and a small positive signal is
applied to the + 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
+ input of Amp1, negative feedback is provided which closes
the loop. An input resistor (RI) makes the circuit look like a
conventional inverting op amp configuration with differential
The gain of this circuit from input to either output is ±RF/RI, or
the single-ended-to-differential gain is 2 × RF/RI. 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.
The scope photo in Figure 40 shows a 10 MHz, 2 V p-p input
signal driving the circuit with 50 m of CAT-5 twisted pair wire.
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.
REFCOM DVSS AVSS AVSS
27 25 16
Figure 41. AD8042 Differential Driver for the AD9220 12-Bit, 10-MSPS ADC
Rev. D | Page 14 of 16