AD637
MAGNITUDE OF ERROR – % OF rms LEVEL
2.0
1.8
1.6
1.4
1.2
CF = 10
1.0
0.8
CF = 7
0.6
DB CALIBRATION
1.
2.
3.
4.
Set V
IN
= 1.00 V dc or 1.00 V rms
Adjust R1 for 0 dB out = 0.00 V
Set V
IN
= 0.1 V dc or 0.10 V rms
Adjust R2 for dB out = – 2.00 V
Any other dB reference can be used by setting V
IN
and R1
accordingly.
LOW FREQUENCY MEASUREMENTS
0.4
0.2
0.0
0.5
1.0
V
IN
– V rms
1.5
2.0
CF = 3
Figure 13. Error vs. RMS Input Level for Three Common
Crest Factors
the energy is contained in the peaks). The duty cycle and peak
amplitude were varied to produce crest factors from l to 10
while maintaining a constant 1 volt rms input amplitude.
CONNECTION FOR dB OUTPUT
Another feature of the AD637 is the logarithmic or decibel out-
put. The internal circuit which computes dB works well over a
60 dB range. The connection for dB measurement is shown in
Figure 14. The user selects the 0 dB level by setting R1 for the
proper 0 dB reference current (which is set to exactly cancel the
log output current from the squarer/divider circuit at the desired
0 dB point). The external op amp is used to provide a more
convenient scale and to allow compensation of the +0.33%/°C
temperature drift of the dB circuit. The special T.C. resistor R3
is available from Tel Labs in Londenderry, New Hampshire
(model Q-81) and from Precision Resistor Inc., Hillside, N.J.
(model PT146).
If the frequencies of the signals to be measured are below
10 Hz, the value of the averaging capacitor required to deliver
even 1% averaging error in the standard rms connection be-
comes extremely large. The circuit shown in Figure 15 shows an
alternative method of obtaining low frequency rms measure-
ments. The averaging time constant is determined by the prod-
uct of R and C
AV1
, in this circuit 0.5 s/µF of C
AV
. This circuit
permits a 20:1 reduction in the value of the averaging capacitor,
permitting the use of high quality tantalum capacitors. It is
suggested that the two pole Sallen-Key filter shown in the dia-
gram be used to obtain a low ripple level and minimize the value
of the averaging capacitor.
If the frequency of interest is below 1 Hz, or if the value of the
averaging capacitor is still too large, the 20:1 ratio can be
increased. This is accomplished by increasing the value of R. If
this is done it is suggested that a low input current, low offset
voltage amplifier like the AD548 be used instead of the internal
buffer amplifier. This is necessary to minimize the offset error
introduced by the combination of amplifier input currents and
the larger resistance.
R2
SIGNAL
INPUT
33.2k
5k
+V
S
BUFFER
BUFFER INPUT
1
ABSOLUTE
VALUE
dB SCALE
FACTOR
ADJUST
AD637
14
13
BUFFER
OUTPUT
SIGNAL
INPUT
*
1k
R3
60.4
AD707JN
COMPENSATED
dB OUTPUT
+ 100mV/dB
–V
S
NC 2
ANALOG COM
OUTPUT
OFFSET
3
BIAS
SECTION
4
12 NC
SQUARER/DIVIDER
25k
10 –V
S
25k
9
FILTER
8
RMS OUTPUT
+
1 F
11 +V
S
CHIP
SELECT 5
DENOMINATOR
6
INPUT
dB
7
C
AV
10k
+V
S
R1
500k
+2.5 VOLTS
*1k + 3500ppm
TC RESISTOR TEL LAB Q81
PRECISION RESISTOR PT146
OR EQUIVALENT
AD508J
0dB ADJUST
Figure 14. dB Connection
–8–
REV. E
AD [ ANALOG DEVICES ]
