OPA376 Datasheet, PDF (11/23 Pages) Burr-Brown (TI) – Precision, Low Noise, Low Quiescent Current, Operational Amplifier

www.ti.com
CAPACITIVE LOAD AND STABILITY
The OPA376 series of amplifiers may be used in
applications where driving a capacitive load is
required. As with all op amps, there may be specific
instances where the OPAx376 can become unstable,
leading to oscillation. The particular op amp circuit
configuration, layout, gain and output loading are
some of the factors to consider when establishing
whether an amplifier will be stable in operation. An op
amp in the unity-gain (+1V/V) buffer configuration and
driving a capacitive load exhibits a greater tendency
to be unstable than an amplifier operated at a higher
noise gain. The capacitive load, in conjunction with
the op amp output resistance, creates a pole within
the feedback loop that degrades the phase margin.
The degradation of the phase margin increases as
the capacitive loading increases.
The OPAx376 in a unity-gain configuration can
directly drive up to 250pF pure capacitive load.
Increasing the gain enhances the ability of the
amplifier to drive greater capacitive loads (see the
typical characteristic plot, Small-Signal Overshoot vs
Capacitive Load. In unity-gain configurations,
capacitive load drive can be improved by inserting a
small (10Ω to 20Ω) resistor, R_S, in series with the
output, as shown in Figure 25. This resistor
significantly reduces ringing while maintaining dc
performance for purely capacitive loads. However, if
there is a resistive load in parallel with the capacitive
load, a voltage divider is created, introducing a gain
error at the output and slightly reducing the output
swing. The error introduced is proportional to the ratio
R_S/R_L, and is generally negligible at low output
current levels.
V+
R_S
OPA376
V_OUT
V_IN
10W to
20W
R_L
C_L
OPA376
OPA2376
OPA4376
SBOS406B – JUNE 2007 – REVISED SEPTEMBER 2007
ACTIVE FILTERING
The OPA376 series is well-suited for filter
applications requiring a wide bandwidth, fast slew
rate, low-noise, single-supply operational amplifier.
Figure 26 shows a 50kHz, 2nd-order, low-pass filter.
The components have been selected to provide a
maximally-flat Butterworth response. Beyond the
cutoff frequency, roll-off is –40dB/dec. The
Butterworth response is ideal for applications
requiring predictable gain characteristics such as the
anti-aliasing filter used ahead of an analog-to-digital
converter (ADC).
R₃
5.49kW
C₂
150pF
R₁
5.49kW
R₂
12.4kW
C₁
V_IN
1nF
V+
OPA376
V_OUT
(V+)/2
Figure 26. Second-Order Butterworth 50kHz
Low-Pass Filter
DRIVING AN ANALOG-TO-DIGITAL
CONVERTER
The low noise and wide gain bandwidth of the
OPA376 family make it an ideal driver for ADCs.
Figure 27 illustrates the OPA376 driving an
ADS8327, 16-bit, 250kSPS converter. The amplifier is
connected as a unity-gain, noninverting buffer.
Figure 25. Improving Capacitive Load Drive
Copyright © 2007, Texas Instruments Incorporated
Submit Documentation Feedback
11
Product Folder Link(s): OPA376 OPA2376 OPA4376