Datasheet

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SBOS278B − SEPTEMBER 2003 − REVISED JANUARY 2004
www.ti.com
12
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors
is used to achieve rail-to-rail output. This output stage is
capable of driving heavy loads connected to any point
between V+ and V−. For light resistive loads ( > 100kΩ ),
the output voltage can swing to 150mV (175mV for dual)
from the supply rail, while still maintaining excellent
linearity (A
OL
> 110dB). With 1kΩ (2kΩ for dual) resistive
loads, the output is specified to swing to within 250mV from
the supply rails with excellent linearity (see the Typical
Characteristics curve Output Voltage Swing vs Output
Current).
CAPACITIVE LOAD AND STABILITY
Capacitive load drive is dependent upon gain and the
overshoot requirements of the application. Increasing the
gain enhances the ability of the amplifier to drive greater
capacitive loads (see the Typical Characteristics curve
Small-Signal Overshoot vs Capacitive Load).
One method of improving capacitive load drive in the
unity-gain configuration is to insert a 10Ω to 20Ω resistor
inside the feedback loop, as shown in Figure 3. This
reduces ringing with large capacitive loads while
maintaining DC accuracy.
R
S
20
Ω
OPA725
C
L
R
L
V
IN
V
OUT
V+
Figure 3. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
DRIVING FAST 16-BIT ADCs
The OPA725 series is optimized for driving fast 16-bit
ADCs such as the ADS8342. The OPA725 op amps buffer
the converter input capacitance and resulting charge
injection, while providing signal gain. Figure 4 shows the
OPA725 in a single-ended method of interfacing to the
ADS8342 16-bit, 250kSPS, 4-channel ADC with an input
range of ±2.5V. The OPA725 has demonstrated excellent
settling time to the 16-bit level within the 600ns acquisition
time of the ADS8342. The RC filter, shown in Figure 4, has
been carefully tuned for best noise and settling
performance. It may need to be adjusted for different op
amp configurations. Please refer to the ADS8342 data
sheet (available for download at www.ti.com) for additional
information on this product.
75
Ω
OPA725
ADS8342
16−Bit ADC
AIN
Common
330pF
V
IN
±
2.5V
+5V
−5V
+5V
−
5V
Figure 4. OPA725 Driving an ADC
TRANSIMPEDANCE AMPLIFIER
Wide bandwidth, low input bias current, and low input
voltage and current noise make the OPA725 an ideal
wideband photodiode transimpedance amplifier. Low-
voltage noise is important because photodiode capaci-
tance causes the effective noise gain of the circuit to
increase at high frequency.
The key elements to a transimpedance design, as shown
in Figure 5, are the expected diode capacitance (C
D
),
which should include the parasitic input common-mode
and differential-mode input capacitance (4pF + 5pF for the
OPA725); the desired transimpedance gain (R
F
); and the
GBW for the OPA725 (20MHz). With these three variables
set, the feedback capacitor value (C
F
) can be set to control
the frequency response. C
F
includes the stray capacitance
of R
F
, which is 0.2pF for a typical surface-mount resistor.
OPA725
V
OUT
10M
Ω
+5V
−
5V
C
D
R
F
C
F
(1)
<1pF
λ
NOTE: (1) C
F
is optional to prevent gain peaking.
It includes the stray capacitance of R
F
.
Figure 5. Dual-Supply Transimpedance Amplifier