_______________Detailed Description

The MAX913 (single) and MAX912 (dual) high-speed

comparators have a unique design that prevents oscil-

lation when the comparator is in its linear region. No

minimum input slew rate is required.

Many high-speed comparators oscillate in the linear

region,

as

shown

in

the

Typical Operating

Characteristics’ industry-standard 686 response graph.

One way to overcome this oscillation is to sample the

output after it has passed through the unstable region.

Another practical solution is to add hysteresis. Either

solution results in a loss of resolution and bandwidth.

Because the MAX912/MAX913 do not need hysteresis,

they offer high resolution to all signals—including low-

frequency signals.

The MAX912/MAX913 provide a TTL-compatible latch

function that holds the comparator output state (Figure 1).

As long as Latch Enable (LE) is high or floating, the input

signal has no effect on the output state. With LE low, the

outputs are controlled by the input differential voltage and

the latch is transparent.

Input Amplifier

A comparator can be thought of as having two sec-

tions: an input amplifier and a logic interface. The

MAX912/MAX913’s input amplifier is fully differential,

with input offset voltage trimmed to below 2.0mV at

+25°C. Input common-mode range extends from

200mV below the negative supply rail to 1.5V below the

positive power supply. The total common-mode range

is 8.7V when operating from ±5VDC supplies.

The MAX912/MAX913’s amplifier has no built-in hys-

teresis. For highest accuracy, do not add hysteresis.

Figure 2 shows how hysteresis degrades resolution.

Resolution

A comparator’s ability to resolve small signal differ-

ences—its resolution—is affected by various factors.

As with most amplifiers, the most significant factors are

and power-supply rejection ratios (CMRR, PSRR). If

source impedance is high, input offset current can be

significant. If source impedance is unbalanced, the

input bias current can introduce another error.

For high-speed comparators, an additional factor in reso-

lution is the comparator’s stability in its linear region. Many

high-speed comparators are useless in their linear region

because they oscillate. This makes the differential input

Hysteresis does not cure the problem, but acts to keep

the input away from its linear range (Figure 2).

The MAX912/MAX913 do not oscillate in the linear region,

which greatly enhances the comparator’s resolution.

__________Applications Information

Power Supplies and Bypassing

The MAX912/MAX913 are tested with ±5V power sup-

of 8.7V (-5.2V to +3.5V). Operation from a single +5V

supply provides a common-mode input range of 3.7V

(-0.2V to +3.5V). Connect V- to GND for single-supply

operation. The MAX912/MAX913 will operate from a

minimum single-supply voltage of +4.5V.

The V+ supply provides power to both the analog input

stage and digital output circuits, whereas the V- supply

only powers the analog section. Bypass V+ and V- to

ground with 0.1µF to 1.0µF ceramic capacitors in parallel

with 10µF or greater tantalum capacitors. Connect the

ceramic capacitors very close to the MAX912/MAX913’s

Single/Dual, Ultra-Fast, Low-Power,

Precision TTL Comparators

_______________________________________________________________________________________

7

Q

Q

(DIFFERENTIAL)

LATCH

ENABLE (LE)

Figure 1. Timing Diagram