Datasheet

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RAIL-TO-RAIL OUTPUT
75W
OPA727
ADS8342
16-BitADC
AIN
Common
330pF
V
IN
±2.5V
+5V
-5V
+5V
-5V
TRANSIMPEDANCE AMPLIFIER
CAPACITIVE LOAD AND STABILITY
R
S
20W
OPA727
C
L
R
L
V
IN
V
OUT
V+
DRIVING FAST 16-BIT ADCs
OPA727
V
OUT
10MW
+5V
-5V
C
D
R
F
C
F
(1)
<1pF
l
NOTE:(1)C isoptionaltopreventgainpeaking.
F
Itincludesthestraycapacitanceof .
F
R
OPA727 , , OPA2727
OPA4727 , OPA728
SBOS314H – SEPTEMBER 2004 – REVISED APRIL 2007
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 (>100k Ω ), the output voltage can
swing to 150mV from the supply rail, while still
maintaining excellent linearity (A
OL
> 110dB). With
1k Ω resistive loads, the output is specified to swing
to within 250mV from the supply rails with excellent
Figure 31. OPA727 Driving an ADC
linearity (see the Typical Characteristics curve,
Output Voltage Swing vs Output Current ).
Wide bandwidth, low input bias current, and low
Capacitive load drive is dependent upon gain and
input voltage and current noise make the OPA727 an
the overshoot requirements of the application.
ideal wideband photodiode transimpedance amplifier.
Increasing the gain enhances the ability of the
Low-voltage noise is important because photodiode
amplifier to drive greater capacitive loads (see the
capacitance causes the effective noise gain of the
Typical Characteristics curve, Small-Signal
circuit to increase at high frequency.
Overshoot vs Capacitive Load ).
The key elements to a transimpedance design, as
One method of improving capacitive load drive in the
shown in Figure 32 , are the expected diode
unity-gain configuration is to insert a 10 Ω to 20 Ω
capacitance (C
D
), which should include the parasitic
resistor inside the feedback loop, as shown in
input common-mode and differential-mode input
Figure 30 . This reduces ringing with large capacitive
capacitance (4pF + 5pF for the OPA727); the desired
loads while maintaining DC accuracy.
transimpedance gain (R
F
); and the GBW for the
OPA727 (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.
Figure 30. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
The OPA727 series is optimized for driving fast
16-bit ADCs such as the ADS8342 . The OPA727 op
amps buffer the converter input capacitance and
resulting charge injection, while providing signal gain.
Figure 31 shows the OPA727 in a single-ended
method of interfacing to the ADS8342 16-bit,
250kSPS, 4-channel ADC with an input range of
± 2.5V. The OPA727 has demonstrated excellent
settling time to the 16-bit level within the 600ns
acquisition time of the ADS8342. The RC filter,
shown in Figure 31 , has been carefully tuned for best
noise and settling performance. It may need to be
Figure 32. Dual-Supply Transimpedance
adjusted for different op amp configurations. Refer to
Amplifier
the ADS8342 data sheet (available for download at
www.ti.com ) for additional information on this
product.
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