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參數(shù)資料
| 型號： | SA572 |
| 廠商： | NXP Semiconductors N.V. |
| 英文描述： | Programmable Analog Compandor(可編程模擬壓縮擴(kuò)展器) |
| 中文描述： | 可編程模擬Compandor（可編程模擬壓縮擴(kuò)展器） |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 136K |
| 代理商： | SA572 |
Philips Semiconductors
Product specification
SA572
Programmable analog compandor
1998 Nov 03
5
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
1
-Q
2
and Q
3
-Q
4
are both tied to the output and
inputs of OPA A
1
. The negative feedback through Q
1
holds the V
BE
of Q
1
-Q
2
and the V
BE
of Q
3
-Q
4
equal. The following relationship can
be derived from the transistor model equation in the forward active
region.
V
BEQ3Q4
BEQ1Q2
(V
BE
= V
T
I
IN
IC/IS)
V
T
I
n
1
G
1
O
I
S
V
T
I
n
1
G
1
O
I
S
where I
IN
V
IN
R
1
R
1
= 6.8k
I
1
= 140
μ
A
I
2
= 280
μ
A
V
T
I
n
I
1
I
IN
I
S
V
T
I
n
I
2
I
1
I
S
I
IN
(2)
where I
IN
V
IN
R
1
R
1
= 6.8k
I
1
= 140
μ
A
I
2
= 280
μ
A
I
O
is the differential output current of the gain cell and I
G
is the gain
control current of the gain cell.
If all transistors Q
1
through Q
4
are of the same size, equation (2)
can be simplified to:
I
O
2
I
2
I
IN
I
G
1
I
2
I
2
2I
1
I
G
(3)
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
μ
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
μ
V in the audio spectrum (10Hz-20kHz). 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
REF
THD
V+
R1
1
1
I
1
140
μ
A
280
μ
A
I
2
I
G
I
O
Q
4
Q
3
Q
1
Q
2
V
IN
+
–
A1
SR00697
Figure 4. Basic Gain Cell Schematic
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