Rainbow Electronics MAX9945 - Manual (Page 7)

Detailed Description
The MAX9945 features a combination of low input cur-
rent and voltage noise, rail-to-rail output voltage swing,
wide supply voltage range, and low-power operation.
The MOS inputs on the MAX9945 make it ideal for use
as transimpedance amplifiers and high-impedance
sensor interface front-ends in medical and industrial
applications. The MAX9945 can interface with small
signals from either current-sources or high-output
impedance voltage sources. Applications include pho-
todiode pulse oximeters, pH sensors, capacitive pres-
sure sensors, chemical analysis equipment, smoke
detectors, and humidity sensors.
A high 130dB open-loop gain (typ) and a wide supply
voltage range, allow high signal-gain implementations
prior to signal conditioning circuitry. Low quiescent
supply current makes the MAX9945 compatible with
portable systems and applications that operate under
tight power budgets. The combination of excellent THD,
low voltage noise, and MOS inputs also make the
MAX9945 ideal for use in high-performance active fil-
ters for data acquisition systems and audio equipment.
Low-Current, Low-Noise Input Stage
The MAX9945 features a MOS-input stage with only
50fA (typ) of input bias current and a low 1fA/√Hz (typ)
input current-noise density. The low-frequency input
voltage noise is a low 2µV
P-P (typ). The input stage
accepts a wide common-mode range, extending from
the negative supply, V
EE,
to within 1.2V of the positive
supply, V CC .
Rail-to-Rail Output Stage
The MAX9945 output stage swings to within 50mV (typ)
of either power-supply rail with a 100k
Ω load and pro-
vides a 3MHz GBW with a 2.2V/µs slew rate. The
device is unity-gain stable, and unlike other devices
with a low quiescent current, can drive a 120pF capaci-
tive load without compromising stability.
Applications Information
High-Impedance Sensor Front Ends
High-impedance sensors can output signals of interest
in either current or voltage form. The MAX9945 inter-
faces to both current-output sensors such as photo-
diodes and potentiostat sensors, and high-impedance
voltage sources such as pH 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 feed-
back capacitors help stabilize the amplifier by cancel-
ing any zeros in the transfer function created by a
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 photodi-
ode dark current if DC accuracy is important. The high
bandwidth and slew rate also allows AC signal pro-
cessing in certain medical photodiode sensor applica-
tions such as pulse oximetry.
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
_______________________________________________________________________________________ 7
Pin Description
PIN
TDFN-EP µMAX
NAME FUNCTION
1 6 OUT Amplifier Output
2 4 V EE
Negative Power Supply. Bypass V EE with 0.1µF ceramic and 4.7µF electrolytic
capacitors to quiet ground plane if different from V EE.
3 3 IN+ Noninverting Amplifier Input
4 2 IN- Inverting Amplifier Input
5 1, 5, 8 N.C. No Connection. Not internally connected.
6 7 V
CC
Positive Power Supply. Bypass V CC with 0.1µF ceramic and 4.7µF electrolytic capacitors
to quiet ground plane or V EE .
— — EP
Exposed Pad. Connect to V EE externally. Connect to a large copper plane to maximize
thermal performance. Not intended as an electrical connection (TDFN only).