High-Speed, Low-Power Voltage Comparators

8

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Applications Information

Circuit Layout

Because of the large gain-bandwidth transfer function of

the MAX900–MAX903, special precautions must be

taken to realize their full high-speed capability. A printed

circuit board with a good, low-inductance ground plane

is mandatory. All decoupling capacitors (the small

100nF ceramic type is a good choice) should be mount-

ed as close as possible to the power-supply pins.

should be paid to the bandwidth of the decoupling and

terminating components. Short lead lengths on the

inputs and outputs are essential to avoid unwanted par-

asitic feedback around the comparators. Solder the

device directly to the printed circuit board instead of

using a socket.

Input Slew-Rate Requirements

As with all high-speed comparators, the high gain-band-

width product of the MAX900–MAX903 can create oscil-

lation problems when the input traverses the linear

region. For clean output switching without oscillation or

steps in the output waveform, the input must meet mini-

mum slew-rate requirements. Oscillation is largely a

function of board layout and of coupled source imped-

ance and stray input capacitance. Both poor layout and

large-source impedance will cause the part to oscillate

and increase the minimum slew-rate requirement. In

some applications, it may be helpful to apply some posi-

tive feedback between the output and + input. This

pushes the output through the transition region cleanly,

but applies a hysteresis in threshold seen at the input

terminals.

TTL Output and Latch Inputs

The comparator TTL-output stages are optimized for dri-

ving low-power Schottky TTL with a fan-out of four.

When the latch is connected to a logic high level or left

floating, the comparator is transparent and immediately

responds to changes at the input terminals. When the

latch is connected to a TTL low level, the comparator

output latches in the same state as at the instant that the

latch command is applied, and will not respond to sub-

sequent changes at the input. No latch is provided on

the MAX901.

Power Supplies

The MAX900–MAX903 can be powered from separate

analog and digital supplies or from a single +5V supply.

The analog supply can range from +5V to +10V with

digital supply always requires +5V.

In high-speed, mixed-signal applications where a com-

mon ground is shared, a noisy digital environment can

adversely affect the analog input signal. When set up

with

separate

supplies

(Figure

1C),

the

MAX900–MAX903 isolate analog and digital signals by

+10V

+5V

OUT

GND

+5V

OUT

GND

+5V

-5V

+5V

OUT

GND

Figure 1A. Separate Analog Supply,

Common Ground

Figure 1B. Single +5V Supply, Common

Ground

Figure 1C. Split ±5V Supply, Separate

Ground

Typical Power-Supply Alternatives