AD636
resemble low duty cycle pulse trains, such as those occurring in
switching power supplies and SCR circuits, have high crest
factors. For example, a rectangular pulse train with a 1% duty
cycle has a crest factor of 10 (C.F. = 1
η
).
Figure 13 is a curve of reading error for the AD636 for a 200 mV
rms input signal with crest factors from 1 to 7. A rectangular
pulse train (pulse width 200
µs)
was used for this test since it is
the worst-case waveform for rms measurement (all the energy is
contained in the peaks). The duty cycle and peak amplitude
were varied to produce crest factors from 1 to 7 while maintain-
ing a constant 200 mV rms input amplitude.
0.5
T
INCREASE IN ERROR – % of Reading
0
0
V
P
EO
= DUTY CYCLE =
CF = 1/
E
IN
(rms) = 200mV
200 s
200 s
T
Circuit Description
The input voltage, V
IN
, is ac coupled by C4 while resistor R8,
together with diodes D1, and D2, provide high input voltage
protection.
The buffer’s output, Pin 6, is ac coupled to the rms converter’s
input (Pin 1) by capacitor C2. Resistor, R9, is connected between
the buffer’s output, a Class A output stage, and the negative output
swing. Resistor R1, is the amplifier’s “bootstrapping” resistor.
With this circuit, single supply operation is made possible by
setting “ground” at a point between the positive and negative
sides of the battery. This is accomplished by sending 250
µA
from the positive battery terminal through resistor R2, then
through the 1.2 volt AD589 bandgap reference, and finally back
to the negative side of the battery via resistor R10. This sets
ground at 1.2 volts +3.18 volts (250
µA ×
12.7 kΩ) = 4.4 volts
below the positive battery terminal and 5.0 volts (250
µA ×
20 kΩ)
above the negative battery terminal. Bypass capacitors C3 and
C5 keep both sides of the battery at a low ac impedance to
ground. The AD589 bandgap reference establishes the 1.2 volt
regulated reference voltage which together with resistor R3 and
trimming potentiometer R4 set the zero dB reference current I
REF
.
Performance Data
–0.5
–1.0
1
2
3
4
5
CREST FACTOR
6
7
Figure 13. Error vs. Crest Factor
A COMPLETE AC DIGITAL VOLTMETER
0 dB Reference Range = 0 dBm (770 mV) to –20 dBm
(77 mV) rms
0 dBm = 1 milliwatt in 600
Ω
Input Range (at I
REF
= 770 mV) = 50 dBm
Input Impedance = approximately 10
10
Ω
V
SUPPLY
Operating Range +5 V dc to +20 V dc
I
QUIESCENT
= 1. 8 mA typical
Accuracy with 1 kHz sine wave and 9 volt dc supply:
0 dB to –40 dBm
±
0.1 dBm
0 dBm to –50 dBm
±
0.15 dBm
+10 dBm to –50 dBm
±
0.5 dBm
Frequency Response 3 dBm
Input
0 dBm = 5 Hz to 380 kHz
–10 dBm = 5 Hz to 370 kHz
–20 dBm = 5 Hz to 240 kHz
–30 dBm = 5 Hz to 100 kHz
–40 dBm = 5 Hz to 45 kHz
–50 dBm = 5 Hz to 17 kHz
Calibration
Figure 14 shows a design for a complete low power ac digital
voltmeter circuit based on the AD636. The 10 MΩ input
attenuator allows full-scale ranges of 200 mV, 2 V, 20 V and
200 V rms. Signals are capacitively coupled to the AD636 buffer
amplifier, which is connected in an ac bootstrapped configura-
tion to minimize loading. The buffer then drives the 6.7 kΩ
input impedance of the AD636. The COM terminal of the ADC
chip provides the false ground required by the AD636 for single
supply operation. An AD589 1.2 volt reference diode is used to
provide a stable 100 millivolt reference for the ADC in the lin-
ear rms mode; in the dB mode, a 1N4148 diode is inserted in
series to provide correction for the temperature coefficient of the
dB scale factor. Calibration of the meter is done by first adjust-
ing offset pot R17 for a proper zero reading, then adjusting the
R13 for an accurate readout at full scale.
Calibration of the dB range is accomplished by adjusting R9 for
the desired 0 dB reference point, then adjusting R14 for the
desired dB scale factor (a scale of 10 counts per dB is convenient).
Total power supply current for this circuit is typically 2.8 mA
using a 7106-type ADC.
A LOW POWER, HIGH INPUT IMPEDANCE dB METER
Introduction
1. First calibrate the zero dB reference level by applying a 1 kHz
sine wave from an audio oscillator at the desired zero dB
amplitude. This may be anywhere from zero dBm (770 mV
rms – 2.2 volts p-p) to –20 dBm (77 mV rms 220 mV – p-p).
Adjust the I
REF
cal trimmer for a zero indication on the analog
meter.
2. The final step is to calibrate the meter scale factor or gain.
Apply an input signal –40 dB below the set zero dB reference
and adjust the scale factor calibration trimmer for a 40
µA
reading on the analog meter.
The temperature compensation resistors for this circuit may be
purchased from:
Tel Labs Inc.,
154 Harvey Road, P.O. Box 375,
Londonderry, NH 03053, Part #Q332A 2 kΩ 1% +3500 ppm/°C
or from
Precision Resistor Company,
109 U.S. Highway 22, Hill-
side, NJ 07205, Part #PT146 2 kΩ 1% +3500 ppm/°C.
The portable dB meter circuit featured here combines the func-
tions of the AD636 rms converter, the AD589 voltage reference,
and a
µA776
low power operational amplifier. This meter offers
excellent bandwidth and superior high and low level accuracy
while consuming minimal power from a standard 9 volt transis-
tor radio battery.
In this circuit, the built-in buffer amplifier of the AD636 is used
as a “bootstrapped” input stage increasing the normal 6.7 kΩ
input Z to an input impedance of approximately 10
10
Ω.
REV. B
–7–
AD [ ANALOG DEVICES ]
