Power-Supply Bypassing

Power-supply bypass capacitors are not required if the

supply impedance is low. For single-supply applications,

capacitor to ground. Do not bypass the reference output.

________________Application Circuits

Low-Frequency Radio Receiver for

Alarms and Detectors

Figure 4’s circuit is useful as a front end for low-frequen-

cy RF alarms. The unshielded inductor (M7334-ND from

resonant circuit to provide frequency selectivity. The op

amp from a MAX952 amplifies the signal received. The

comparator improves noise immunity, provides a signal

strength threshold, and translates the received signal

into a pulse train. Carrier frequencies are limited to

around 10kHz. 10kHz is used in the example in Figure 4.

The layout and routing of components for the amplifier

should be tight to minimize 60Hz interference and

crosstalk from the comparator. Metal shielding is rec-

ommended to prevent RFI from the comparator or digi-

tal circuitry from exciting the receiving antenna. The

transmitting antenna can be long parallel wires spaced

about 7.2cm apart, with equal but opposite currents.

Radio waves from this antenna will be detectable when

the receiver is brought within close proximity, but can-

cel out at greater distances.

Infrared Receiver Front End for

Remote Controls and Data Links

The circuit in Figure 5 uses the MAX952 as a PIN pho-

todiode preamplifier and discriminator for an infrared

receiver. The op amp is configured as a Delyiannis-

Friend bandpass filter to reduce disturbances from

noise and eliminate low-frequency interference from

sunlight, fluorescent lights, etc. This circuit is applica-

ble for TV remote controls and low-frequency data links

up to 20kbps. Carrier frequencies are limited to around

10kHz. 10kHz is used in the example circuit.

Component layout and routing for the amplifier should

be tight to reduce stray capacitance, 60Hz interfer-

ence, and RFI from the comparator. Crosstalk from

comparator edges will distort the amplifier signal. In

order to minimize the effect, a lowpass RC filter is

added to the connection from the reference to the non-

inverting input of the op amp.

Sensor Preamp and Alarm Trigger for

Smoke Detectors

The high-impedance CMOS inputs of the MAX951–

MAX954 op amp are ideal for buffering high-imped-

ance sensors, such as smoke detector ionization cham-

bers, piezoelectric transducers, gas detectors, and pH

sensors. Input bias currents are typically less than 3pA

at room temperature. A 5µA typical quiescent current

for the MAX953 will minimize battery drain without

resorting to complex sleep schemes, allowing continu-

ous monitoring and immediate detection.

Ionization-type smoke detectors use a radioactive source,

such as Americium, to ionize smoke particles. A positive

voltage on a plate attached to the source repels the posi-

tive smoke ions and accelerates them toward an outer

electrode connected to ground. Some ions collect on an

intermediate plate. With careful design, the voltage on this

plate will stabilize at a little less than one-half the supply

voltage under normal conditions, but rise higher when

smoke increases the ion current. This voltage is buffered

Ultra-Low-Power, Single-Supply

Op Amp + Comparator + Reference

______________________________________________________________________________________

11

REF

0.1

µF

0.1

µF

1

2

10M

100k

1.2V

30k

COMP

AMP

C2

15pF, 5%

10kHz,

5Vp-p

C1

150pF,

5%

NEC

PH302B

NEC

SE307-C

51

Ω

49.9k

1%

49.9k

1%

R2

1.0M,

1%

LAYOUT-SENSITIVE AREA

MAX952

R1 x C1 = R2 x C2 =

5.1M

LAYOUT-SENSITIVE AREA

RADIOACTIVE

IONIZATION

CHAMBER

SMOKE SENSOR

COMP

4.7M

AMP

MAX953

Figure 5. Infrared Receiver Application

Figure 6. Sensor Preamp and Alarm Trigger Application