SA572 Philips, SA572 Datasheet - Page 5

• Manufacturer Part Number: SA572
• Description: Programmable analog compandor
• Manufacturer: Philips

Philips Semiconductors
SA572 BASIC APPLICATIONS
Description
The SA572 consists of two linearized, temperature-compensated
gain cells ( G), each with a full-wave rectifier and a buffer amplifier
as shown in the block diagram. The two channels share a 2.5V
common bias reference derived from the power supply but otherwise
operate independently. Because of inherent low distortion, low noise
and the capability to linearize large signals, a wide dynamic range
can be obtained. The buffer amplifiers are provided to permit control
of attack time and recovery time independent of each other.
Partitioned as shown in the block diagram, the IC allows flexibility in
the design of system levels that optimize DC shift, ripple distortion,
tracking accuracy and noise floor for a wide range of application
requirements.
Gain Cell
Figure 4 shows the circuit configuration of the gain cell. Bases of the
differential pairs Q
inputs of OPA A
of Q
be derived from the transistor model equation in the forward active
region.
(V
V
where I
1998 Nov 03
V
T
BE
I
Programmable analog compandor
BE
n
1
-Q
= V
Q3Q4
2
1
2
IN
T
R
I
I
and the V
I
1
2
G
I
1
IN
= 140 A
= 280 A
= 6.8k
I
S
IC/IS)
V
R
1
2
1
IN
BE
1
I
. The negative feedback through Q
O
1
Q1Q2
-Q
BE
2
of Q
and Q
V
3
T
-Q
I
n
3
4
-Q
I
equal. The following relationship can
O
1
2
4
are both tied to the output and
I
G
Q
I
S
I
4
G
1
2
I
O
Q
3
1
2
1
I
G
Figure 4. Basic Gain Cell Schematic
holds the V
TRIM
1
2
THD
I
O
BE
+
5
A1
V
REF
V
where I
I
control current of the gain cell.
If all transistors Q
can be simplified to:
I
The first term of Equation 3 shows the multiplier relationship of a
linearized two quadrant transconductance amplifier. The second
term is the gain control feedthrough due to the mismatch of devices.
In the design, this has been minimized by large matched devices
and careful layout. Offset voltage is caused by the device mismatch
and it leads to even harmonic distortion. The offset voltage can be
trimmed out by feeding a current source within 25 A into the THD
trim pin.
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 100mV, the gain cell gives THD (total harmonic
distortion) of 0.17% typ. Output noise with no input signals is only
6 V in the audio spectrum (10Hz-20kHz). The output current I
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
I
O
O
O
T
–
I
is the differential output current of the gain cell and I
is DC coupled.
n
I
I
2
2
1
IN
140 A
R
I
I
1
2
I
I
R1
6.8k
S
1
1
= 140 A
= 280 A
I
= 6.8k
I
IN
IN
V
V
IN
R
IN
1
1
V+
I
G
through Q
V
280 A
T
I
Q
n
I
1
2
I
1
2
I
I
2
2
4
are of the same size, equation (2)
Q
I
2I
1
I
2
S
1
SR00697
I
IN
I
G
REF
(2)
if the output current
Product specification
(3)
SA572
G
is the gain
O