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MAX951

Ultra-Low-Power, Single-Supply Op Amp + Comparator + Reference

Maxim Integrated 

MAX951–MAX954

Ultra-Low-Power, Single-Supply

Op Amp + Comparator + Reference

AMPIN+

AMPOUT

R2

R1

2pF to 10pF

Figure 3. Compensation for Feedback-Node Capacitance

R

A

≈

R2 VSHYST ,

VDD

for

V SHYST

>> VIHYST

In the example, RA is again 24kΩ.

6) Select the upper trip point VS(H). Our example is set at

4.75V.

7) Calculate RB.

( ) RB =

(VREF + VIHYST) (R2)(R A )

(R2) VS(H) − (VREF + VIHSYT)(R A + R2)

where RB is 8.19kΩ, or approximately 8.2kΩ.

Input Noise Considerations

Because low power requirements often demand high-

impedance circuits, effects from radiated noise are more

significant. Thus, traces between the op amp or compara-

tor inputs and any resistor networks attached should be

kept as short as possible.

Crosstalk

Reference

Internal crosstalk to the reference from the comparator is

package dependent. Typical values (VDD = 5V) are 45mV

for the plastic DIP package and 32mV for the SO pack-

age. Applications using the reference for the op amp or

external circuitry can eliminate this crosstalk by using a

simple RC lowpass filter, as shown in Figure 5.

Op Amp

Internal crosstalk to the op amp from the comparator is

package dependent, but not input-referred. Typical val-

ues (VDD = 5V) are 4mV for the plastic DIP package and

280μV for the SO package.

ANTENNA

VCC = 5V

L1

330mH

C1A

390pF

C1B

330pF

L1 x C1 =

1

(2�fC)2

0.1µF

MAX952

AMP

0.1µF

20kΩ

10MΩ

C1C

20pF to

60pF

1MΩ

5.1MΩ

100kΩ 1.2V COMP

REF

LAYOUT-SENSITIVE AREA,

METAL RFI SHIELDING ADVISED

Figure 4. Low-Frequency Radio Receiver Application

Op Amp Stability and Board Layout

Considerations

Unlike other industry-standard micropower CMOS op

amps, the op amps in the MAX951–MAX954 maintain

stability in their minimum gain configuration while driving

heavy capacitive loads, as demonstrated in the MAX951/

MAX953 Op Amp Percent Overshoot vs. Capacitive Load

graph in the Typical Operating Characteristics.

Although this family is primarily designed for low-frequen-

cy applications, good layout is extremely important. Low-

power, high-impedance circuits may increase the effects

of board leakage and stray capacitance. For example, the

combination of a 10MΩ resistance (from leakage between

traces on a contaminated, poorly designed PC board) and

a 1pF stray capacitance provides a pole at approximately

16kHz, which is near the amplifier’s bandwidth. Board

routing and layout should minimize leakage and stray

capacitance. In some cases, stray capacitance may be

unavoidable and it may be necessary to add a 2pF to 10pF

capacitor across the feedback resistor to compensate;

select the smallest capacitor value that ensures stability.

Input Overdrive

With 100mV overdrive, comparator propagation delay is

typically 6μs. The Typical Operating Characteristics show

propagation delay for various overdrive levels.

Supply current can increase when the op amp in the

MAX951–MAX954 is overdriven to the negative supply

rail. For example, when connecting the op amp as a com-

parator and applying a -100mV input overdrive, supply

current rises by around 15μA and 32μA for supply volt-

ages of 2.8V and 7V, respectively.

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