SA572
The residual distortion is third harmonic distortion and
is caused by gain control ripple. In a compandor system,
available control of fast attack and slow recovery improve
ripple distortion significantly. At the unity gain level of
100 mV, the gain cell gives THD (total harmonic
distortion) of 0.17% typ. Output noise with no input signals
is only 6.0
mV
in the audio spectrum (10 Hz-20 kHz). The
output current I
O
must feed the virtual ground input of an
operational amplifier with a resistor from output to
inverting input. The non-inverting input of the operational
amplifier has to be biased at V
REF
if the output current I
O
is DC coupled.
V+
1
1
I
)
I
2
G
2
O
I
1
140mA
I
O
A1
+
Q
4
Q
3
−
Q
1
Q
2
R
1
6.8kW
I
G
THD
TRIM
V
REF
V
IN
I
2
280mA
Figure 3. Basic Gain Cell Schematic
Rectifier
V+
I
+
V IN
*
V REF
R2
The rectifier is a full-wave design as shown in Figure 4.
The input voltage is converted to current through the input
resistor R
2
and turns on either Q
5
or Q
6
depending on the
signal polarity. Deadband of the voltage to current
converter is reduced by the loop gain of the gain block A
2
.
If AC coupling is used, the rectifier error comes only from
input bias current of gain block A
2
. The input bias current
is typically about 70 nA. Frequency response of the gain
block A
2
also causes second-order error at high frequency.
The collector current of Q
6
is mirrored and summed at the
collector of Q
5
to form the full wave rectified output
current I
R
. The rectifier transfer function is:
I
R
+
V
IN
*
V
REF
R
2
(eq. 3)
R
V
REF
+
A2
−
Q
5
D
7
Q
6
R
2
V
IN
If V
IN
is AC-coupled, then the equation will be reduced
to:
I
RAC
+
V
IN
(AVG)
R
2
The internal bias scheme limits the maximum output
current I
R
to be around 300
mA.
Within a
"1.0
dB error
band the input range of the rectifier is about 52 dB.
Figure 4. Simplified Rectifier Schematic
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