AD636J Analog Devices, AD636J Datasheet - Page 6

• Manufacturer Part Number: AD636J
• Description: Low Level/ True RMS-to-DC Converter
• Manufacturer: Analog Devices

AD636
The current mirror also produces the output current, I
which equals 2I
voltage with R2 and buffered by A
voltage output. The transfer function of the AD636 thus results:
The dB output is derived from the emitter of Q
age at this point is proportional to –log V
Q
voltage is zero when the externally supplied emitter current
(I
THE AD636 BUFFER AMPLIFIER
The buffer amplifier included in the AD636 offers the user
additional application flexibility. It is important to understand
some of the characteristics of this amplifier to obtain optimum
performance. Figure 10 shows a simplified schematic of the buffer.
Since the output of an rms-to-dc converter is always positive, it
is not necessary to use a traditional complementary Class AB
output stage. In the AD636 buffer, a Class A emitter follower is
used instead. In addition to excellent positive output voltage
swing, this configuration allows the output to swing fully down
to ground in single-supply applications without the problems
associated with most IC operational amplifiers.
When this amplifier is used in dual-supply applications as an
input buffer amplifier driving a load resistance referred to
ground, steps must be taken to insure an adequate negative
voltage swing. For negative outputs, current will flow from the
load resistor through the 40 k emitter resistor, setting up a
voltage divider between –V
–V
V
Figure 10. AD636 Buffer Amplifier Simplified Schematic
IN
5
REF
, buffers and level shifts this voltage, so that the dB output
S
, will limit the available negative output swing of the buffer.
1
) to Q
10k
R3
VOLTAGE –CURRENT
BUFFER
ABSOLUTE VALUE/
5
INPUT
CONVERTER
approximates I
20k
A1
R4
8k
Figure 9. Simplified Schematic
4
. I
V
OUT
OUT
+
8k
can be used directly or converted to a
5 A
CURRENT
MIRROR
|
V
R4
A2
2 R2 I rms
IN
+V
|
S
3
S
5 A
.
and ground. This reduced effective
–V
I
1
S
ONE-QUADRANT
Q1
(OPTIONAL, SEE TEXT)
4
SQUARER/
10k
40k
DIVIDER
to provide a low impedance
Q2
10 A
R
A3
E
FS
R
Q4
V
CURRENT MIRROR
EXTERNAL
Q3
BUFFER
OUTPUT
IN
IN
25k
I
3
. Emitter follower,
R1
rms
I
4
4
C
Q5
AV
BUF
3
IN
, since the volt-
7
I
OUT
R
8
I
LOAD
BUFFER
REF
20 A
FS
A4
10k
10k
OUT
R2
,
14
10
9
5
6
3
COM
R
dB
OUT
OUT
+V
BUF
–V
L
S
S
–6–
Addition of an external resistor in parallel with R
voltage divider such that increased negative swing is possible.
Figure 11 shows the value of R
V
increases the quiescent current of the buffer amplifier by an
amount equal to R
with no R
FREQUENCY RESPONSE
The AD636 utilizes a logarithmic circuit in performing the
implicit rms computation. As with any log circuit, bandwidth is
proportional to signal level. The solid lines in the graph below
represent the frequency response of the AD636 at input levels
from 1 millivolt to 1 volt rms. The dashed lines indicate the
upper frequency limits for 1%, 10%, and 3 dB of reading
additional error. For example, note that a 1 volt rms signal will
produce less than 1% of reading additional error up to 220 kHz.
A 10 millivolt signal can be measured with 1% of reading addi-
tional error (100 V) up to 14 kHz.
AC MEASUREMENT ACCURACY AND CREST FACTOR
Crest factor is often overlooked in determining the accuracy of
an ac measurement. Crest factor is defined as the ratio of the
peak signal amplitude to the rms value of the signal (C.F. = V
V rms) Most common waveforms, such as sine and triangle
waves, have relatively low crest factors (<2). Waveforms that
PEAK
Figure 11. Ratio of Peak Negative Swing to –V
R
EXTERNAL
to –V
200m
100m
100
30m
10m
1.0
0.5
1m
0
1
EXTERNAL
0
1k
Figure 12. AD636 Frequency Response
S
1 VOLT rms INPUT
200mV rms INPUT
100mV rms INPUT
1mV rms INPUT
30mV rms INPUT
10mV rms
for Several/Load Resistances
for several values of R
INPUT
EXT
is 30 A at –V
1k
10k
/–V
S
. Nominal buffer quiescent current
FREQUENCY – Hz
R
EXTERNAL
EXTERNAL
10k
100k
S
LOAD.
1%
R
= –5 V.
L
= 6.7k
–
Addition, of R
for a particular ratio of
10%
R
R
100k
L
L
1M
= 16.7k
= 50k
E
3dB
alters this
S
EXTERNAL
vs.
1M
10M
REV. B
P
/