This method is recommended for additional hysteresis of up

to a few hundred millivolts. Beyond that, the impedance of

R3 is low enough to affect the bias string and adjustment of

R1 may be also required.

Circuit Layout and Bypassing

The LMV7219 requires high-speed layout. Follow these lay-

out guidelines:

1. Power supply bypassing is critical, and will improve stabil-

ity and transient response. A decoupling capacitor such as

0.1µF ceramic should be placed as close as possible to V

+

pin. An additional 2.2µF tantalum capacitor may be required

for extra noise reduction.

2. Keep all leads short to reduce stray capacitance and lead

inductance. It will also minimize unwanted parasitic feedback

around the comparator.

3. The device should be soldered directly to the PC board in-

stead of using a socket.

4. Use a PC board with a good, unbroken low inductance

ground plane. Make sure ground paths are low-impedance,

especially were heavier currents are flowing.

5. Input traces should be kept away from output traces. This

can be achieved by running a topside ground plane between

the output and inputs.

6. Run the ground trace under the device up to the bypass

capacitor to shield the inputs from the outputs.

7. To prevent parasitic feedback when input signals are

slow-moving, a small capacitor of 1000pF or less can be

placed between the inputs. It can also help eliminate oscilla-

tions in the transition region. However, this capacitor can

cause some degradation to tpd when the source impedance

is low.

Zero-Crossing Detector

The inverting input is connected to ground and the non-

inverting input is connected to 100mVp-p signal. As the sig-

nal at the non-inverting input crosses 0V, the comparator’s

output Changes State.

Threshold Detector

Instead of tying the inverting input to 0V, the inverting input

can be tied to a reference voltage. The non-inverting input is

connected to the input. As the input passes the V

old, the comparator’s output changes state.

Crystal Oscillator

A simple crystal oscillator using the LMV7219 is shown be-

low. Resistors R1 and R2 set the bias point at the compara-

tor’s non-inverting input. Resistors R3, R4 and C1 sets the

inverting input node at an appropriate DC average level

based on the output. The crystal’s path provides resonant

positive feedback and stable oscillation occurs. The output

duty cycle for this circuit is roughly 50%, but it is affected by

resistor tolerances and to a lesser extent by the comparator

offset.

DS101054-21

FIGURE 2. Additional Hysteresis

DS101054-22

FIGURE 3. Zero-Crossing Detector

DS101054-23

FIGURE 4. Threshold Detector

DS101054-24

FIGURE 5. Crystal Oscillator

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