OPA4830 Datasheet, PDF (29/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

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