MAX9637 Maxim, MAX9637 Datasheet - Page 10

• Manufacturer Part Number: MAX9637
• Description: The MAX9636/MAX9637/MAX9638 are single-supply, CMOS input op amps featuring wide bandwidth at low quiescent current, making them suitable for a broad range of battery-powered applications such as portable medical instruments, portable media players,
• Manufacturer: Maxim

Figure 1. Using a Series Resistor to Isolate the Capacitive
Load from the Op Amp
3V/5V Low-Power, Low-Noise, CMOS,
Rail-to-Rail I/O Op Amps
The ICs have a high tolerance for capacitive loads.
In unity-gain configuration, the op amps can typically
drive up to 300pF pure capacitive load. Increasing the
gain enhances the amplifier’s ability to drive greater
capacitive loads. In unity-gain configurations, capacitive
load drive can be improved by inserting a small (5I to
30I) isolation resistor, R
as shown in Figure 1. This significantly reduces ringing
while maintaining DC performance for purely capaci-
tive loads. However, if the load also has a resistive
component then a voltage-divider is created, introduc-
ing a direct current (DC) error at the output. The error
introduced is proportional to the ratio RISO/RL, which
is usually negligible in most cases. Applications that
cannot tolerate this slight DC error can use an alterna-
tive approach of providing stability by placing a suitable
resistance in parallel with the capacitive load as shown
in Figure 2 (see the Typical Operating Characteristics
section for graphs of the stable operating region for
various capacitive loads vs. resistive loads). While this
approach of adding a resistor parallel to the load does
not introduce DC error, it nevertheless reduces the out-
put swing proportionally.
Figure 3. The MAX9636 in a Sensor Preamp Configuration
10
+
-
MAX9636/
MAX9637/
MAX9638
A
V
=
R
L
PHOTODIODE
Driving Capacitive Loads
+ R
R
L
ISO
ISO
, in series with the output,
≈ 1V/V
R
ISO
R
REF
L
IN+
IN-
C
MAX9636
L
V
DD
OUT
Figure 2. Using a Parallel Resistor to Degenerate the Effect of
the Capacitive Load and Increase Stability
The ICs interface to both current-output sensors, such
as photodiodes (Figure 3), and high-impedance voltage
sources, such as piezoelectric sensors. For current-
output sensors, a transimpedance amplifier is the most
noise-efficient method for converting the input signal to
a voltage. High-value feedback resistors are commonly
chosen to create large gains, while feedback capacitors
help stabilize the amplifier by cancelling any poles intro-
duced in the feedback function by the highly capacitive
sensor or cabling. A combination of low-current noise
and low-voltage noise is important for these applications.
Take care to calibrate out photodiode dark current if DC
accuracy is important. The high bandwidth and slew
rate also allows AC signal processing in certain medical
photodiode sensor applications such as pulse oximetry.
For voltage-output sensors, a noninverting amplifier is
typically used to buffer and/or apply a small gain to the
input voltage signal. Due to the extremely high imped-
ance of the sensor output, a low input bias current with
minimal temperature variation is very important for these
applications.
CONDITIONING/
-
+
MAX9636/
MAX9637/
FILTERS
MAX9638
SIGNAL
MAX1286
Sensor Front-Ends
ADC
High-Impedance
R
P
R
L
C
L