SSM2018TS AD [Analog Devices], SSM2018TS Datasheet - Page 14

• Manufacturer Part Number: SSM2018TS
• Description: Trimless Voltage Controlled Amplifiers
• Manufacturer: AD [Analog Devices]

SSM2018T/SSM2118T
Figure 42. Two TCRs Compensate for Temperature Drift
of Gain Constant
Figure 43. Current Source Allows Temperature Compen-
sation with One TCR
One of the resistors in the divider to the control port can be sub-
stituted with an appropriately chosen TCR to compensate the
SSM2018T or the SSM2118T as shown in Figure 42. Because
the resistor divider effectively cuts the temperature coefficient in
half, two TCRs must be used. The combined drift of the two is
7000 ppm/ C, given an effective drift for to the control voltage
of –3500 ppm/ C. Of course, a single TCR with the appropriate
coefficient can be used. The 3500 ppm parts were chosen be-
cause they are a standard item and do not need to be special
ordered.
In many applications, an op amp is used to drive the control
voltage. If this is the case, it may be more economical to use the
op amp and a single TCR for temperature compensation. The
op amp is configured as a Howland current source as shown in
Figure 43. The current then flows through a single TCR to
create the control voltage. Because the resistor divider is not
present, the temp coefficient is equivalent to the TCR’s coef-
ficient. Using this technique, the drift was reduced from
–3500 ppm/ C to –150 ppm/ C, which results in a total com-
pensated gain shift of 0.4 dB over the full temperature range at a
gain of 20 dB.
Digital Control of the Gain
A common method of controlling the gain of a VCA is to use a
digital-to-analog converter to set the control voltage. Figure 44
shows a 12-bit DAC, the DAC8512, controlling the SSM2018T
(or SSM2118T). The DAC8512 is a complete 12-bit converter
in an 8-pin package. It includes an on board reference and a
output amplifier to produce an output voltage from 0 V to
+4.095 V, which is 1 mV/bit. Since the voltage is always posi-
tive, this circuit only provides attenuation. The resistor divider
on the output of the DAC8512 is set to scale the output voltage
CONTROL
VOLTAGE
CONTROL
VOLTAGE
10k
R1
2k
R2
10k
OP176
1k *
3500ppm/ C
1k *
3500ppm/ C
10k
50pF
9k
R3
R5
+15V
–15V
1µF
R4
1k
1k *
3500ppm/ C
V
*PRECISION RESISTOR CO.
10601 75
LARGO, FL 34647
[phone]
C
(PIN 11)
V
SSM2018T OR SSM2118T
C
(PIN 11)
SSM2018T OR SSM2118T
TH
ST. NORTH
–14–
such that full scale produces 80 dB of attenuation. The resistor
divider can be adjusted to provide other attenuation ranges. If a
parallel interface is needed, then the DAC8562 may be used, or
for a dual DAC, the AD8582.
Supply Considerations and Single Supply Operation
The SSM2018T and SSM2118T have a wide operating supply
range. Many of the graphs in this data sheet show the perfor-
mance of the part from 5 V to 18 V. These graphs offer typi-
cal performance specifications and are a good indication of the
parts capabilities. The minimum operating supply voltage is
account for supply variations, the recommended minimum sup-
ply is 5 V.
The circuits in the data sheet do not show supply decoupling for
simplicity; however, to ensure best performance, each supply
pin should be decoupled with a 0.1 F ceramic (or other low re-
sistance and inductance type) capacitor as close to the package
as possible. This minimizes the chance of supply noise feeding
through the part and causing excessive noise in the audio fre-
quency range.
The SSM2018T and SSM2118T can be operated in single sup-
ply mode as long as the circuit is properly biased. Figure 45
shows the proper configuration, which includes an amplifier to
create a false ground node midway between the supplies. A
high quality, wide bandwidth audio amplifier such as the OP176
or AD797 should be used to ensure a very low impedance
ground over the full audio frequency range. The minimum op-
erating supply for the SSM2018 is 5 V, which gives a mini-
mum single supply of +10 V and ground. The performance of
the circuit with +10 V is identical to graphs that show operation
of the SSM2018T with 5 V supplies.
4.5 V. Below this voltage, the parts are inoperable. Thus, to
+15V
V
IN
Figure 44. 12-Bit DAC Controls the VCA Gain
0.1 F
18k
SCLK
CLR
SDI
CS
LD
2
6
4
5
3
47pF
NC
NC
DAC8512
4
+5V
1
2
3
5
6
7
8
1
7
SSM2018T
50pF
18k
0.1 F
NC = NO CONNECT
8
825
16
15
14
13
12
11
10
R6
9
NC
NC
1k
R7
0V
150k
V
C
0.1 F
+2.24V
C
1 F
V
+15V
–15V
CON
REV. A
OUT