Download THAT Corporation Design Note 137

1
THAT Corporation Design Note 137
Substituting THAT 2180- and 2181-Series VCAs
for THAT 2150-Series VCAs in Existing Designs
Abstract
THAT Corporation’s 2180- and 2181-series VCAs are pin-for-pin compatible, improved
performance replacements for the 2150-series VCAs. Designers may convert existing
2150-series designs to use the 2180/2181 VCAs without making any changes to existing PCB
layouts. This application note details the few component value changes and component
deletions required to obtain optimum performance with the 2180- and 2181-series VCAs in
existing 2150-series designs.
Choosing the correct VCA
Table 1 shows the correct VCA to use when switching from a THAT 2150-series part to one of
the THAT 218x-series. When retrofitting the 218x VCAs, designers may replace the
215x-series VCA with a trimmable VCA or a trimless VCA. For high performance applications,
the choice is normally to choose a trimmable 2181Lx or 2181Sx. However, where distortion
performance is less critical and trim-free production is a goal, the 2180Lx is the right choice.
Old VCA
THAT 2155
THAT 2150A
THAT 2151
Precision
THAT 2181LC or THAT2181SC
THAT 2181LB or THAT2181SB
THAT 2181LA or THAT2181SA
Trimless
THAT 2180LC
THAT 2180LB
THAT 2180LA
Table 1
2180 Series
The 2180-series VCAs are pre-trimmed at wafer stage for low THD and control-voltage
feedthrough without further adjustment. The parts are available in three grades (2180LA,
2180LB, and 2180LC) selected for factory-trimmed distortion performance, allowing the
user to optimize cost vs. performance.
Figure 1 shows a typical 2150-series application circuit in which the negative control port
(pin 3) is used for gain control. Components R2, R3, and VR1 provide for external adjustment
of VCA symmetry. Figure 2 shows the same circuit adapted for use with a 2180-series VCA.
Note that external symmetry trim components R2, R3, and VR1 have been deleted. In
particular, R2 must not be used with the 2180 Series—it will upset the internal symmetry
trim. Capacitor C3 may be reduced in value to as low as 22 pF in order to increase the circuit
bandwidth if desired.
Figures 3 and 5 show similar 2150-series application circuits wherein the positive control
port (pins 2 and 4) or both control ports are used, respectively. Figures 4 and 6 show the
2180-series counterparts to these circuits.
Copyright © 2004 by THAT Corporation . All rights reserved.
Document 600057 Rev 00
2
THAT Corporation Design Note 137
Substituting the 218x for the 215x
Table 2 summarizes the component deletions required to convert a typical 2150 application
to the 2180-series.
External
Component
R2
R3
VR1
2150 Series
2151
51
470k
50k
2150A
51
390k
50k
2180 Series
2155
51
300k
50k
2180A
DELETE
DELETE
DELETE
2180B
DELETE
DELETE
DELETE
2180C
DELETE
DELETE
DELETE
Table 2
2181 Series
The 2181-series VCAs are selected after packaging primarily on the basis of externally
trimmed THD and control-voltage feedthrough performance. The parts are selected into
three grades (2181A, 2181B, and 2181C) based on 100% tested performance after external
trim. External trimming of VCA symmetry allows higher performance than can be
consistently obtained with pretrimmed parts. Both SIP (L designation) and SO-8
(S designation) packages are available in the 2181-series. Of course, since the 2150-series
was only available in SIP, only that package applies to the retrofit situation.
Figures 7, 8, and 9 illustrate typical 2181 application circuits utilizing the negative, positive,
and both control ports respectively. Note that the external trim potentiometer VR1, and its
wiper resistor R3 are present as in the 2150 circuits, but that R2 is deleted. R2 is not
required because there is an internal 27 ohm resistor between pins 2 and 4 in all 2181-series
VCAs. The values shown for R3 are scaled differently, based on +/- 15V supplies, for each of
the three grades of these parts.
As with the 2180-series circuits, C3 may be reduced to as low as 22 pF if desired.
Table 3 summarizes the component value changes required to convert 2150-series circuits
to utilize the 2181-series VCAs (assuming V+ = +15V, V- = -15V, see the 2181 datasheet to
calculate R3 values for other supply voltages).
External
Component
R2
R3
VR1
2150 Series
2151
51
470k
50k
2150A
51
390k
50k
2181 Series
2155
51
300k
50k
2181A
DELETE
680k
50k
2181B
DELETE
220k
50k
2181C
DELETE
130k
50k
Table 3
T H AT Corporation
45 Sumner Street, Milford MA 01757-1656
Tel: +1 (508) 478-9200 Fax: +1 (508) 478-0990
Email: [email protected] Web: www.thatcorp.com
3
THAT Corporation Design Note 137
Substituting the 218x for the 215x
V+
V+
C2
2150A-390k
2151-470k
2155-300k
VR1
50k
100n
R3
R2
51R
R5
V-
20k0
C3
2
C1
Input
U1
4
EC+
7
SYM
R1
1
20k0
GND
EC-
2150A, 2151
or 2155
8
U2A
2
OUT
IN
47u
47p
V+
Output
1
V-
3
5
4570
6
3
R4
5k1
Control Voltage
V-
Figure 1. Typical 2150-series application circuit with Ec- driven
V+
C2
R5
100n
20k0
C3
2
4
EC+
Input
C1
R1
7
SYM
V+
1
20k0
GND
EC-
2180A, 2180B
or 2180C
47p
8
OUT
IN
47u
U1
V-
2
U2A
1
Output
3
4570
5
6
3
R4
5k1
Control Voltage
V-
Figure 2. Typical 2180-series application circuit with Ec- driven
Copyright © 2004 by THAT Corporation . All rights reserved.
Document 600057 Rev 00
4
THAT Corporation Design Note 137
Substituting the 218x for the 215x
V+
V+
2150A-390k
2151-470k
2155-300k
VR1
50k
C2
100n
R3
R2
51R
V-
R5
20k0
C3
Control Voltage
2
4
EC+
C1
Input
R1
V+
1
47p
8
U2A
2
OUT
IN
47u
U1
7
SYM
20k0
V-
GND
EC-
Output
1
3
4570
5
6
2150A, 2151
or 2155
R4
5k1
3
V-
Figure 3. Typical 2150-series application circuit with Ec+ driven
V+
C2
100n
R5
Control Voltage
20k0
C3
2
4
EC+
Input
C1
R1
7
SYM
V+
1
20k0
GND
EC-
2180A, 2180B
or 2180C
8
2
OUT
IN
47u
47p
U1
V-
U2A
1
Output
3
4570
5
6
R4
5k1
3
V-
Figure 4. Typical 2180-series application circuit with Ec+ driven
T H AT Corporation
45 Sumner Street, Milford MA 01757-1656
Tel: +1 (508) 478-9200 Fax: +1 (508) 478-0990
Email: [email protected] Web: www.thatcorp.com
5
THAT Corporation Design Note 137
Substituting the 218x for the 215x
V+
V+
2150A-390k
2151-470k
2155-300k
VR1
50k
C2
100n
R3
R2
51R
V-
R5
20k0
C3
Pos. Control Voltage
2
4
EC+
C1
Input
R1
V+
1
20k0
GND
EC-
2150A, 2151
or 2155
47p
8
2
U2A
OUT
IN
47u
U1
7
SYM
Output
1
V-
3
5
4570
6
R4
5k1
3
Neg. Control Voltage
V-
Figure 5. Typical 2150-series application circuit with both control pins driven
V+
C2
100n
R5
Pos. Control Voltage
20k0
C3
2
4
EC+
Input
C1
R1
7
SYM
V+
1
20k0
GND
EC-
2180A, 2180B
or 2180C
47p
8
2
OUT
IN
47u
U1
V-
U2A
1
Output
3
4570
5
6
3
R4
5k1
Neg. Control Voltage
V-
Figure 6. Typical 2180-series application circuit with both control pins driven
Copyright © 2004 by THAT Corporation . All rights reserved.
Document 600057 Rev 00
6
THAT Corporation Design Note 137
V+
Substituting the 218x for the 215x
V+
2181A-680k
2181B-220k
2181C-130k
VR1
50k
C2
100n
R3
R5
20k0
C3
V2
4
EC+
Input
C1
R1
7
SYM
V+
1
OUT
20k0
GND
EC-
2181A, 2181B
or 2181C
47p
8
IN
47u
U1
U2A
2
Output
1
V-
3
4570
5
6
R4
5k1
3
Control Voltage
V-
Figure 7. Typical 2181-series application circuit with Ec- driven
V+
V+
2181A-680k
2181B-220k
2181C-130k
VR1
50k
C2
100n
R3
R5
V-
20k0
C3
Control Voltage
2
4
EC+
Input
C1
R1
7
SYM
V+
1
20k0
GND
EC-
2181A, 2181B
or 2181C
47p
8
OUT
IN
47u
U1
V-
2
U2A
1
Output
3
4570
5
6
R4
5k1
3
V-
Figure 8. Typical 2181-series application circuit with Ec+ driven
T H AT Corporation
45 Sumner Street, Milford MA 01757-1656
Tel: +1 (508) 478-9200 Fax: +1 (508) 478-0990
Email: [email protected] Web: www.thatcorp.com
7
THAT Corporation Design Note 137
V+
VR1
50k
Substituting the 218x for the 215x
V+
C2
2181A-680k
2181B-220k
2181C-130k
100n
R3
R5
V-
20k0
C3
Pos. Control Voltage
2
4
EC+
Input
C1
R1
7
SYM
V+
1
20k0
GND
EC-
2181A, 2181B
or 2181C
47p
8
OUT
IN
47u
U1
V-
2
U2A
1
Output
3
5
4570
6
R4
5k1
3
Neg. Control Voltage
V-
Figure 9. Typical 2181-series application circuit with both control pins driven
Copyright © 2004 by THAT Corporation . All rights reserved.
Document 600057 Rev 00
8
THAT Corporation Design Note 137
Substituting the 218x for the 215x
Notes:
T H AT Corporation
45 Sumner Street, Milford MA 01757-1656
Tel: +1 (508) 478-9200 Fax: +1 (508) 478-0990
Email: [email protected] Web: www.thatcorp.com