Circuit detects rapidly falling signals and rejects noise
Combine an op-amp and a logic IC to determine signal peaks beyond a set threshold.
Vladimir Rentyuk, Zaporozhye, Ukraine; Edited by Paul Rako and Fran Granville -- EDN, November 3, 2011
Detecting a rapidly falling signal over some threshold is important for ultrasonic or location equipment as well as for seismology systems. You can combine a rail-to-rail operational amplifier with a Schmitt-trigger logic gate to perform this function (Figure 1). This example works well in an ultrasound machine. It controls a sample-and-hold amplifier that sets the gain of an AGC (automatic-gain-control) system.
R1 and C1 form an input lowpass
filter to smooth the input signal. You
set the values of R1 and C1 to create a
filter roll-off for the input signal you are
processing. Resistors R3 and R4 establish
a small hysteresis, which is necessary
so that slow signals with noise don’t
cause the output to change state. You set
the threshold level with voltage divider
R6 and R7. D1, R5, and C2 form a peak
detector. R5 establishes a time constant
of the discharge of C2 and provides
sensitivity to a falling signal’s rate. You
establish the circuit’s sensitivity to a
falling signal’s rate of change using the
time constant, which the values of C2
and R5 set. Hysteresis resistor R4 is more
than a decade larger than R5, so the
effect of resistors R3 and R4 is negligible.
A rising input signal greater than the
threshold charges C2 to approximately
the level of the input signal. The output
amplifier is at a high level because the voltage on C2 is always lower than the
value of the rising input signal due to
D1’s voltage drop. When the input drops
faster than C2 can discharge through R5,
the output level of the device changes to
a low level because the voltage on C2 is
higher than the value of the falling input
signal. If the input signal falls more slowly
than the discharge of C2 through resistor
R5, the output remains high. Schottky
diode D1 prevents the discharge of C2
through the input. R2 and D2 clamp the
amplifier’s output to positive values. Feed
the clamped signal to Schmitt-trigger
logic gate IC2 to give a logic-level output
with fast transitions (Figure 2).Talkback
