comparator is used with the reference, the combined

peak-to-peak noise is about 1mV. This, of course, is

much higher than the RMS noise of the individual

components. Care should be taken in the layout to

avoid capacitive coupling from any output to the

reference pin. Crosstalk can significantly increase the

actual noise of the reference.

__________Applications Information

Hysteresis

Hysteresis increases the comparators’ noise margin by

increasing the upper threshold and decreasing the

lower threshold (see Figure 2).

Hysteresis (MAX921/MAX923)

To add hysteresis to the MAX921 or MAX923, connect

resistor R1 between REF and HYST, and connect

resistor R2 between HYST and V- (Figure 3). If no

hysteresis is required, connect HYST to REF. When

hysteresis is added, the upper threshold increases by

the same amount that the lower threshold decreases.

The hysteresis band (the difference between the upper

twice the voltage between REF and HYST. The HYST

input can be adjusted to a maximum voltage of REF

and to a minimum voltage of (REF – 50mV). The

maximum difference between REF and HYST (50mV)

will therefore produce a 100mV max hysteresis band.

Use the following equations to determine R1 and R2:

should not exceed the REF source capability, and

should be significantly larger than the HYST input

usually appropriate. If 2.4M

approximated as:

When hysteresis is obtained in this manner for the

MAX923, the same hysteresis applies to both comparators.

Hysteresis (MAX922/MAX924)

Hysteresis can be set with two resistors using positive

feedback, as shown in Figure 4. This circuit generally

draws more current than the circuits using the HYST

pin on the MAX921 and MAX923, and the high

feedback impedance slows hysteresis. The design

procedure is as follows:

1. Choose R3. The leakage current of IN+ is under

1nA (up to +85°C), so the current through R3 can be

around 100nA and still maintain good accuracy.

or 100nA for R3 = 11.8M

Ω. 10MΩ is a good

practical value.

between the upper and lower thresholds. In this

3. Calculate R1.

5. Calculate R2.

A 1% preferred value is 64.9k

Ω.

R2 =

V

(V

1

R1

1

R3

3

(1.182

100k)

1

100k

1

10M

65.44k

THR

REF

R1)

1

1

×











 −−

















=

×









−−

















=

Ω

R1 = R3

V

V

10M

0.05

5

100k

HB

×

+

=×

=Ω

R1 (k ) = V

(mV)

HB

Ω

R1 =

V

2

I

R2 =

1.182 –

V

2

I

HB

REF

HB

REF

×

()









Ultra Low-Power,

Single/Dual-Supply Comparators

10

______________________________________________________________________________________

GND

V-

V+

MAX924

OUT

R3

R1

R2

V+

Figure 4. External Hysteresis