OPA4684M Datasheet, PDF (21/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
Figure 51. Low-Power, XDSL Transceiver Design.
dual-channel, differential ADC driver
Where a low-power, single-supply, interface to a differential input +5-V ADC is required, the circuit of
Figure 52 can provide a high dynamic range, medium gain interface for dual high-performance ADCs. The circuit
of Figure 52 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 provides both signal gain, single to differential conversion, and a reduction in noise figure. To
show a 50ĆΩ input impedance at the input to the transformer, two 200ĆΩ resistors are required on the transformer
secondary. These two resistors are also the amplifier gain elements. Since the same DC voltage appears on
both inverting nodes in the circuit of Figure 52, 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 52 is particularly suitable for a moderate resolution dual ADC used as I/Q samplers. The
optional 500ĆΩ 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. Figure 53 shows the harmonic distortion for the circuit
of Figure 52 with and without these pull-down resistors. The 5 mA 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
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