SSM2018TS AD [Analog Devices], SSM2018TS Datasheet - Page 13

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

REV. A
on temperature: V
1.38E-23, q = electron charge = 1.6E-19, and T = absolute
temperature in Kelvin). This temperature dependency leads to
the –3500 ppm/ C drift of the control law. It also means that
the control law changes as the part warms up. Thus, our speci-
fication for the control law states that the part has been powered
up for 60 seconds.
When the part is initially turned on, the temperature of the die
is still at the ambient temperature (25 C for example), but the
power dissipation causes the die to warm up. With 15 V sup-
plies and a supply current of 11 mA, 330 mW is dissipated.
This number is multiplied by
die’s temperature. In this case, the die increases from 25 C to
approximately 50 C. A 25 C temperature change causes a
8.25% increase in the gain constant, resulting in a gain constant
of 30 mV/dB. The graph in Figure 31 shows how the gain con-
stant varies over the full temperature range.
Proper Operating Mode for the SSM2018T and SSM2118T
Both parts have the flexibility of operating in either Class A or
Class AB. This is accomplished by adjusting the amount of cur-
rent flowing in the gain core (I
trade-off between the two classes is that Class A tends to have
lower THD but higher noise than Class AB. However, by utiliz-
ing well matched gain core transistors, distortion compensation
circuitry, and laser trimming, the SSM2018T and SSM2118T
have excellent THD performance in Class AB. Thus, the parts
offer the best of both worlds in having the low noise of Class AB
with low THD.
Because the parts operate optimally in Class AB, the distortion
trim is performed for this class. To guarantee conformance to the
data sheet THD specifications, both the SSM2018T and SSM2118T
must be operated in Class AB. This does not mean that the parts
cannot be operated in Class A, but the optimal THD trim point
is different for the two classes. Using Class A operation results
in a shift of THD performance from a typical value of 0.006%
to 0.05% without trim. An external potentiometer could be
added to change the trim back to its optimal point as shown in
the OVCE application circuit, but this adds the expense and
time in adjusting a potentiometer.
The class of operation is set by selecting the proper value for R
shown in Figure 37. R
MODE input (Pin 12). For class AB operation with 15 V
supplies, R
current remains at 95 A. This current follows the formula:
The factor of 0.7 V arises from the fact that the dc bias on Pin
12 is a diode drop above ground.
Output Drive
The SSM2018T is buffered by an internal op amp to provide a
low impedance output. This output is capable of driving to
within 1.2 V of either rail at 1% distortion for a 100 k load.
(Note: This 100 k load is in parallel with the feedback resistor
of 18 k , so the effective load is 15.3 k .) For better than
0.01% distortion, the output should remain about 3.5 V away
from either rail as shown in Figure 3. As the graph of output
A. For other supply voltages, adjust the value of R
B
should be 150 k . This results in a current of 95
T
I
= kT/q (k = Boltzmann’s constant =
MODE
B
determines the current flowing into the
(V
M
JA
CC
in Figure 38). The traditional
to determine the rise in the
R
– 0.7V )
B
B
such that
B
–13–
swing versus load resistance shows (Figure 10), to maintain less
than 1% distortion, the output current should be limited to
approximately 1.3 mA. If higher current drive is required,
then the output should be buffered with a high quality op amp
such as the OP176 or AD797.
The internal amplifiers are compensated for unity gain stability
and are capable of driving a capacitive load up to 4700 pF.
Larger capacitive loads should be isolated from the output of the
SSM2018T by the use of a 50
Upgrading SSM2018 Sockets
The SSM2018T easily replaces the SSM2018 in the basic VCA
configuration. The parts are pin for pin compatible allowing di-
rect replacement. At the same time, the trimming potentiom-
eters for symmetry and offset should be removed, as shown in
Figure 41. Upgrading to the SSM2018T immediately saves the
expense of the potentiometers and the time in production of
trimming for minimum distortion and control feedthrough.
If the SSM2018 is used in the OVCE or VCP configuration, the
SSM2018T can still directly replace it. However, the potenti-
ometers cannot necessarily be removed, as explained in the
OVCE and VCP sections.
Temperature Compensation of the Gain Constant
As explained above, the gain constant has a 3500 ppm/ C tem-
perature drift due to the inherent nature of the control port.
Over the full temperature range of –40 C to +85 C, the drift
causes the gain to change by 7 dB if the part is in a gain of
same over a wide temperature range, then external temperature
compensation should be employed. The simplest form of com-
pensation is a temperature compensating resistor (TCR), such
as the PT146 from Precision Resistor Co. These elements are
different from a standard thermistor in that they are linear over
temperature to better match the linear drift of the gain constant.
20 dB. If the application requires that the gain constant be the
V
V
IN+
IN–
V+
V–
OFFSET
TRIM
100k
1µF 18k
1µF 18k
R
NC = NO CONNECT
Figure 41. Upgrading SSM2018 Sockets
B
: 150k FOR CLASS AB
V+
10M
REMOVE FOR SSM2018T
47pF
SYMMETRY
TRIM
500k
1
2
3
4
5
6
7
8
SSM2018T
SSM2018T/SSM2118T
470k
18k
50pF
series resistor.
16
14
13
12
11
10
15
9
NC
V–
R
1µF
B
V
OUT
V+
1k
3k
V
CONTROL