SA571 Datasheet, PDF (8/11 Pages) NXP Semiconductors – Compandor

Philips Semiconductors
Compandor
Product specification
SA571
+20
0
MAXIMUM
–20
SIGNAL LEVEL
110dB
–40
90dB
–60
–80
–100
–40
NOISE IN
20kHz BW
–20
0
VCA GAIN (0dB)
Figure 16. Dynamic Range
SR00690
Control signal feedthrough is generated in the gain cell by imperfect
device matching and mismatches in the current sources, I₁ and I₂.
When no input signal is present, changing I_G will cause a small
output signal. The distortion trim is effective in nulling out any control
signal feedthrough, but in general, the null for minimum feedthrough
will be different than the null in distortion. The control signal
feedthrough can be trimmed independently of distortion by tying a
current source to the ∆G input pin. This effectively trims I₁. Figure 17
shows such a trim network.
V_CC
R-SELECT FOR
100k
3.6V
470k
TO PIN 3 OR 14
RESISTORS
Inspection of the gain equations in Figures 7 and 8 will show that the
basic compressor and expander circuit gains may be set entirely by
resistor ratios and the internal voltage reference. Thus, any form of
resistors that match well would suffice for these simple hook-ups,
and absolute accuracy and temperature coefficient would be of no
importance. However, as one starts to modify the gain equation with
external resistors, the internal resistor accuracy and tempco become
very significant. Figure 19 shows the effects of temperature on the
diffused resistors which are normally used in integrated circuits, and
the ion-implanted resistors which are used in this circuit. Over the
critical 0°C to +70°C temperature range, there is a 10-to-1 improve-
ment in drift from a 5% change for the diffused resistors, to a 0.5%
change for the implemented resistors. The implanted resistors have
another advantage in that they can be made the size of the diffused
resistors due to the higher resistivity. This saves a significant
amount of chip area.
140Ω /
1.15
DIFFUSED
RESISTOR
1.10
1kΩ /
ÇÇÇÇÇÇ ^1.05
LOW TC
IMPLANTED
RESISTOR
1.00
1% ERROR
BAND
.95
–40
0 40 80 120
TEMPERATURE
Figure 19. Resistance vs Temperature
SR00693
Figure 17. Control Signal Feedthrough
SR00691
OPERATIONAL AMPLIFIER
The main op amp shown in the chip block diagram is equivalent to a
741 with a 1MHz bandwidth. Figure 18 shows the basic circuit. Split
collectors are used in the input pair to reduce g_M, so that a small
compensation capacitor of just 10pF may be used. The output
stage, although capable of output currents in excess of 20mA, is
biased for a low quiescent current to conserve power. When driving
heavy loads, this leads to a small amount of crossover distortion.
–IN
Q₁
Q₃
I₁
Q₂
Q₄
I₂
+IN
D₁
D₂
C_C
Q₂
Q₆
OUT
Figure 18. Operational Amplifier
SR00692
1997 Aug 14
8