Datasheet
DUAL-CHANNEL, DIFFERENTIAL ADC
VIDEO LINE DRIVING
+5V
1/4
OPA4830
1/4
OPA4830
800W
800W
R
S
R
S
16.7dB
NoiseFigure
Gain=8V/V
18dB
200W
200W
1kW
1kW
0.1 Fm
V
CM
C
L
500W
500W
V
CM
DualADC
1of2
Channels
1:2
50W
Source
OPA4830
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.................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008
signal at the transformer input. The two pull-down
DRIVER resistors do show a signal path ground and should be
connected at the same physical point to ground, in
Where a low-noise, single-supply, interface to a
order to eliminate imbalanced ground return currents
differential input +5V ADC is required, the circuit of
from degrading 2nd-harmonic distortion.
Figure 85 can provide a high dynamic range, medium
gain interface for dual high-performance ADCs. The
circuit of Figure 85 uses two amplifiers in the
differential inverting configuration. The common-mode
Most video distribution systems are designed with
voltage is set on the noninverting inputs to the supply
75 Ω series resistors to drive a matched 75 Ω cable. In
midscale. In this example, the input signal is coupled
order to deliver a net gain of 1 to the 75 Ω matched
in through a 1:2 transformer. This design provides
load, the amplifier is typically set up for a voltage gain
both signal gain, single to differential conversion, and
of +2V/V, compensating for the 6dB attenuation of the
a reduction in noise figure. To show a 50 Ω input
voltage divider formed by the series and shunt 75 Ω
impedance at the input to the transformer, two 200 Ω
resistors at either end of the cable.
resistors are required on the transformer secondary.
The circuit of Figure 72 applies to this requirement if
These two resistors are also the amplifier gain
all references to 50 Ω resistors are replaced by 75 Ω
elements. Because the same dc voltage appears on
values. Often, the amplifier gain is further increased
both inverting nodes in the circuit of Figure 85 , no dc
to 2.2, which recovers the additional dc loss of a
current will flow through the transformer, giving a dc
typical long cable run. This change would require the
gain of 1 to the output for this common-mode voltage,
gain resistor (R
G
) in Figure 72 to be reduced from
V
CM
.
750 Ω to 625 Ω . In either case, both the gain flatness
The circuit of Figure 85 is particularly suitable for a
and the differential gain/phase performance of the
moderate resolution dual ADC used as I/Q samplers.
OPA4830 provide exceptional results in video
The optional 500 Ω resistors to ground on each
distribution applications. Differential gain and phase
amplifier output can be added to improve the 2nd-
measure the change in overall small-signal gain and
and 3rd-harmonic distortion by >15dB if higher
phase for the color sub-carrier frequency (3.58MHz in
dynamic range is required.
NTSC systems) versus changes in the large-signal
output level (which represents luminance information
The 5mA added output stage current significantly
in a composite video signal). The OPA4830, with the
improves linearity if that is required. The measured
typical 150 Ω load of a single matched video cable,
2nd-harmonic distortion is consistently lower than the
shows less than 0.07%/0.17 ° differential gain/phase
3rd-harmonics for this balanced differential design. It
errors over the standard luminance range for a
is particularly helpful for this low-power design if there
positive video (negative sync) signal. Similar
are no grounds in the signal path after the low-level
performance would be observed for multiple video
signals (see Figure 86 ).
Figure 85. Single-Supply Differential ADC Driver (1 of 2 channels)
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