SA572 Datasheet, PDF (7/12 Pages) NXP Semiconductors – Programmable analog compandor

Philips Semiconductors
Programmable analog compandor
Product specification
SA572
Q8 Q9
V+
Q10
Q17
IR
+
VIN
R
10k
–
A3
+
I_R1
D11
D12
I_Q = 2I_R2
I_R2
X2
Q16
10k
D15
D13
Q14
X2
Q18
CR
CA
TRACKING
TRIM
Figure 6. Buffer Amplifier Schematic
SR00699
Basic Expandor
Figure 7 shows an application of the circuit as a simple expandor.
The gain expression of the system is given by
V_OUT
V_IN
+
2
I₁
R₃ @ V_IN(AVG)
@ _{R2 @ R1}
(5)
(I₁=140µA)
Both the resistors R₁ and R₂ are tied to internal summing nodes. R₁
is a 6.8k internal resistor. The maximum input current into the gain
cell can be as large as 140µA. This corresponds to a voltage level of
140µA • 6.8k=952mV peak. The input peak current into the rectifier
is limited to 300µA by the internal bias system. Note that the value
of R₁ can be increased to accommodate higher input level. R₂ and
R₃ are external resistors. It is easy to adjust the ratio of R₃/R₂ for
desirable system voltage and current levels. A small R₂ results in
higher gain control current and smaller static and dynamic tracking
error. However, an impedance buffer A₁ may be necessary if the
input is voltage drive with large source impedance.
The gain cell output current feeds the summing node of the external
OPA A₂. R₃ and A₂ convert the gain cell output current to the output
voltage. In high-performance applications, A₂ has to be low-noise,
high-speed and wide band so that the high-performance output of
the gain cell will not be degraded. The non-inverting input of A₂ can
be biased at the low noise internal reference Pin 6 or 10. Resistor
R₄ is used to bias up the output DC level of A₂ for maximum swing.
The output DC level of A₂ is given by
ǒ Ǔ V_ODC
+ V_REF
1
)
R₃
R₄
*
V_B
R₃
R₄
(6)
V_B can be tied to a regulated power supply for a dual supply system
and be grounded for a single supply system. CA sets the attack time
constant and CR sets the recovery time constant. *5COL
1998 Nov 03
7