ES636
True RMS-to-DC Converters
The dB output is obtained by the voltage at the emitter of Q3, which is proportional to
the -log V
IN
. The emitter follower Q5 buffers and level shifts this voltage so that the dB
output is zero when the externally set emitter current for Q5 approximates I
3
.
V
+
BUFIN
7
+
-
A4
BUFOUT
6
I
3
R1
6K
4
8
I
OUT
R2
10K
R
L
9
Absolute value/voltage-current converter
i
upper
R4
20K
C
AV
I
4
I
1
A3
V
in
R4
Q1
Q3
5
dB
OUT
+
V
in
1
+
-
A1
i
lower
8K
-
+
A2
Q2
Q4
Q5
R3
10K
8K
One-Quadrant
square/divider
V
-
Figure 1. ES636 Simplified Schematic
Standard Connection (Figure 2)
The standard RMS connection requires only one external component, C
AV
. In this
configuration the ES636 measures the RMS of the AC and DC levels present at the
input, but shows an error for low-frequency inputs as a function of the C
AV
filter
capacitor. Figure 3 gives practical values of C
AV
for various values of averaging error
over frequency for the standard RMS connections (no post filtering). If a 3uF capacitor
is chosen, the additional error at 30Hz will be 1%. If the DC error can be rejected, a
capacitor should be connected in series with the input, as would typically be the case in
single-supply operation.
The input and output signal ranges are a function of the supply voltages. Refer to the
electrical characteristics for guaranteed performance. The buffer amplifier can be used
either for lowering the output impedance of the circuit, or for other applications such as
buffering high-impedance input signals. The ES636 can be used in current output mode
by disconnecting the internal load resistor, RL, from ground. The current output is
available at pin 8 with a nominal scale of 100μA/ V
RMS
input for the ES636. The
7
09/02/16
CYRUSTEK [ Cyrustek corporation ]
