OPA4684 Datasheet, PDF (17/26 Pages) Burr-Brown (TI) – Quad, Low-Power, Current-Feedback OPERATIONAL AMPLIFIER

DUAL-CHANNEL, DIFFERENTIAL ADC DRIVER
Where a low-power, single-supply, interface to a differential
input +5V ADC is required, the circuit of Figure 12 can
provide a high dynamic range, medium gain interface for
dual high-performance ADCs. The circuit of Figure 12 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 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. Since the same DC voltage appears
on both inverting nodes in the circuit of Figure 12, 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 12 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. Figure 13 shows
the harmonic distortion for the circuit of Figure 12 with and
without these pull-down resistors. The 5mA added output
stage current significantly improves linearity if that is re-
quired. The measured 2nd-harmonic distortion is consis-
tently lower than the 3rd-harmonics for this balanced differ-
ential 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 to eliminate
imbalanced ground return currents from degrading 2nd-
harmonic distortion.
–50
2Vp-p Output
3rd-Harmonic
–60
2nd-Harmonic
–70
No Pull-Down
–80
3rd-Harmonic
2nd-Harmonic
–90
1
5mA/Ch Pull-Down
10
20
Frequency (MHz)
FIGURE 13. Harmonic Distortion vs Frequency.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two PC boards are available to assist in the initial evaluation
of circuit performance using the OPA4684 in its two package
styles. Both of these are available, free, as an unpopulated
PC board delivered with descriptive documentation. The
summary information for these boards is shown in Table I
below.
PRODUCT
OPA4684ID
OPA4684IDBW
PACKAGE
SO-14
TSSOP-14
BOARD
PART
NUMBER
DEM-OPA468xD
DEM-OPA46xxDPW
LITERATURE
REQUEST
NUMBER
SBOU016
—
TABLE I. Evaluation Module Ordering Information.
+5V
50Ω
Source
V_CM
0.1µF
10kΩ
10kΩ
1/4
OPA4684
200Ω
1:2
800Ω
200Ω
800Ω
500Ω
R_S
R_S
Dual ADC
1 of 2
C_L
Channels
14.7dB
Noise Figure
Gain = 8V/V
18.1dB
1/4
OPA4684
V_CM
FIGURE 12. Single-Supply Differential ADC Driver (1 of 2 Channels).
500Ω
OPA4684
17
SBOS240B
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