SA572 Datasheet, PDF (5/12 Pages) NXP Semiconductors – Programmable analog compandor

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
2
IG
)
1
2
IO
I₁
140mA
I_O
Q₄
Q₃
A1
+
−
Q₁
Q₂
R₁
6.8kW
I_G
I₂
280mA
THD
TRIM
V_REF
V_IN
Figure 3. Basic Gain Cell Schematic
Rectifier
The rectifier is a full-wave design as shown in Figure 4.
The input voltage is converted to current through the input
resistor R₂ and turns on either Q₅ or Q₆ depending on the
signal polarity. Deadband of the voltage to current
converter is reduced by the loop gain of the gain block A₂.
If AC coupling is used, the rectifier error comes only from
input bias current of gain block A₂. The input bias current
is typically about 70 nA. Frequency response of the gain
block A₂ also causes second-order error at high frequency.
The collector current of Q₆ is mirrored and summed at the
collector of Q₅ to form the full wave rectified output
current I_R. The rectifier transfer function is:
I_R
+
V_IN
* V_REF
R₂
(eq. 3)
If V_IN is AC-coupled, then the equation will be reduced
to:
I_RAC
+
V_IN(AVG)
R₂
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.
V_REF
+
A2
−
R₂
V_IN
V+
IR
+
VIN * VREF
R2
Q₅
D₇
Q₆
Figure 4. Simplified Rectifier Schematic
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