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a
FEATURES Transformer-Like Balanced Output Drives 10 V RMS Into a 600 Load Stable When Driving Large Capacitive Loads and Long Cables Low Distortion 0.006% typ 20 Hz-20 kHz, 10 V RMS into 600 High Slew Rate 15 V/ s typ Low Gain Error (Differential or Single-Ended); 0.7% typ Outputs Short-Circuit Protected Available In Space-Saving 8-Pin Mini-DIP Package Low Cost APPLICATIONS Audio Mix Consoles Distribution Amplifiers Graphic and Parametric Equalizers Dynamic Range Processors Digital Effects Processors Telecommunications Systems Industrial Instrumentation Hi-Fi Equipment
VIN
Balanced Line Driver SSM2142
FUNCTIONAL BLOCK DIAGRAM
50 +OUT FORCE
+OUT SENSE 10k - OUT SENSE
50 - OUT FORCE
ALL RESISTORS 30k UNLESS OTHERWISE INDICATED GND
10k
GENERAL DESCRIPTION
The SSM2142 is an integrated differential-output buffer amplifier that converts a single-ended input signal to a balanced output signal pair with high output drive. By utilizing low noise thermally matched thin film resistors and high slew rate amplifiers, the SSM2142 helps maintain the sonic quality of audio systems by eliminating power line hum, RF interference, voltage drops, and other externally generated noise commonly encountered with long audio cable runs. Excellent rejection of common-mode noise and offset errors is achieved by laser trimming of the onboard resistors, assuring high gain accuracy. The carefully designed output stage of the SSM2142 is capable of driving difficult loads, yielding low distortion performance despite extremely long cables or loads as low as 600 , and is stable over a wide range of operating conditions.
Based on a cross-coupled, electronically balanced topology, the SSM2142 mimics the performance of fully balanced transformer-based solutions for line driving. However, the SSM2142 maintains lower distortion and occupies much less board space than transformers while achieving comparable common-mode rejection performance with reduced parts count. The SSM2142 in tandem with the SSM2141 differential receiver establishes a complete, reliable solution for driving and receiving audio signals over long cables. The SSM2141 features an Input Common-Mode Rejection Ratio of 100 dB at 60 Hz. Specifications demonstrating the performance of this typical system are included in the data sheet.
REV. B
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 Fax: 617/326-8703
SSM2142-SPECIFICATIONS noted. Typical characteristics apply to operation at T = +25 C.)
A
(VS =
18 V, -40 C TA +85 C, operating in differential mode unless otherwise
Min Typ 10 750 5.8 5.98 5.94 0.7 60 -38 -35 80 -45 -40 0.006 -93.4 +93.4 15 2 5.7 Max 900 Units k A dB dB % dB dB dB % dBu dBu V/s 250 50 mV mV V 55 7.0 mA mA
Parameter INPUT IMPEDANCE INPUT CURRENT GAIN, DIFFERENTIAL GAIN, SINGLE-ENDED GAIN ERROR, DIFFERENTIAL POWER SUPPLY REJECTION RATIO STATIC OUTPUT COMMON-MODE REJECTION OUTPUT SIGNAL BALANCE RATIO TOTAL HARMONIC DISTORTION Plus Noise SIGNAL-TO-NOISE RATIO HEADROOM SLEW RATE OUTPUT COMMON-MODE VOLTAGE OFFSET1 DIFFERENTIAL OUTPUT VOLTAGE OFFSET DIFFERENTIAL OUTPUT VOLTAGE SWING OUTPUT IMPEDANCE SUPPLY CURRENT OUTPUT CURRENT, SHORT CIRCUIT
Symbol ZIN IIN
Conditions VIN = 7.071 V Single-Ended Mode RL = 600
PSRR OCMR SBR THD+N SNR HR SR VOOS VOOD
VS = 13 V to 18 V See Test Circuit; f = 1 kHz See Test Circuit; f = 1 kHz 20 Hz to 20 kHz, VO = 10 V rms, RL = 600 VIN = 0 V CLIP Level = 10.5 V rms
RL = 600 RL = 600 VIN = 7.071 V
-250 -50 13.8 45
25 15 14.14 50 5.5 70
ZO ISY ISC Unloaded, VIN = 0 V
60
NOTES 1 Output common-mode offset voltage can be removed by inserting dc blocking capacitors in the sense lines. See Applications Information. Specifications subject to change without notice.
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 V Storage Temperature . . . . . . . . . . . . . . . . . . -60C to +150C Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . +300C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150C Operating Temperature Range . . . . . . . . . . . . -40C to +85C Output Short Circuit Duration (Both Outputs) . . . . Indefinite
*Stresses above those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only; the functional operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ABSOLUTE MAXIMUM RATINGS*
PIN CONNECTIONS 8-Pin Plastic DIP (P Suffix) 16-Pin Wide Body SOL (S Suffix)
NC NC - FORCE - SENSE GROUND VIN NC
1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9
NC NC + FORCE + SENSE +V -V NC NC
ORDERING GUIDE
NC
Model
Operating Temperature Range
Package Description Plastic DIP SOL
Package Option N-8 R-16
SSM2142P -40C to +85C SSM2142S* -40C to +85C
*For availability of SOIC package, contact your local sales office.
-2-
REV. B
SSM2142
300 V OUT V
300
1 2 3 VS = 0V 4
8 7 6 5
300
1 2
8 7 6 5 VOUT VIN
300
+18V -18V V CMR = 10V p-p
600
VIN = 10V p-p
3 4
+18V -18V
600
V VOUT
VOUT OCMR = 20 LOG VCMR
SBR = 20 LOG
Figure 1. Output CMR Test Circuit
Figure 2. Signal Balance Ratio (BBC Method) Test Circuit
Typical Performance Characteristics
140 120 TA = +25C VS = 18V VS = 1V 100 -PSR 80 60 +PSR
12 TA = +25C VS = 18V RL = 600 DIFF. MODE 0.1% DISTORTION 0.01% DISTORTION 6
POWER SUPPLY REJECTION - dB
OUTPUT VOLTAGE SWING - V rms
10
8
4
40
20
2
0 10 100 1k FREQUENCY - Hz 10k 100k
0 10 20 30 FREQUENCY - kHz 50 100
Figure 3. Power Supply Rejection vs. Frequency
12 TA = +25C RL = 600
Figure 4. Maximum Output Voltage Swing vs. Frequency
6.5 TA = +25C VIN = 0V NO LOAD 5.5
OUTPUT VOLTAGE SWING - V rms
10
6.0
8 0.1% DISTORTION 6
SUPPLY CURRENT - mA
DIFF. MODE FREQ. = 20kHz
5.0
4
4.5
2
4.0
0 2 6 10 SUPPLY VOLTAGE - Volts 14 18
3.5 2 6 10 SUPPLY VOLTAGE - Volts 14 18
Figure 5. Output Voltage Swing vs. Supply Voltage
Figure 6. Supply Current vs. Supply Voltage
REV. B
-3-
SSM2142
THD PERFORMANCE
The following data, taken from the THD test circuit on an Audio Precision System One using the internal 80 kHz noise filter, demonstrates the typical performance of a balanced pair system based on the SSM2142/SSM2141 chip set. Both differential and single-ended modes of operation are shown, under a number of output load conditions which simulate various application situations. Note also that there is no adverse effect on system performance when using the optional series feedback capacitors, which reject dc cable offsets in order to maintain optimal ac noise rejection. The large signal transient response of the system to a 100 kHz square wave input is also shown, demonstrating the stability of the SSM2142 under load.
+18V
10F* 4 6 7 2 8 1
A C
V IN
SSM 3 2142 5
RL
B
SSM 2141
VOUT
Figure 9. THD+N vs. Frequency at Point B (Differential Mode)
10F*
R1
R2
-18V *USED ONLY IN THD PLOTS AS NOTED. ALL CABLE MEASUREMENTS USE BELDEN 8451 CABLE.
Figure 7. THD Test Circuit
Figure 10. THD+N vs. Frequency at Point A (Single Ended)
Figure 8. THD+N vs. Frequency at Point B (Differential Mode)
Figure 11. THD+N vs. Frequency at Point C (SSM2141 Output)
-4-
REV. B
SSM2142
on-chip 50 series damping resistors. The impedances in the output buffer pair are precisely balanced by laser trimming during production. This results in the high gain accuracy needed to obtain good common-mode noise rejection, and excellent separation between the offset error voltages common to the cable pair and the desired differential input signal. As shown in the test circuit, it is suggested that a suitable balanced, high input-impedance differential amplifier such as the SSM2141 be used at the receiving end for best system performance. The SSM2141 receiver output is configured for a gain of one half following the 6 dB gain of the SSM2142, in order to maintain an overall system gain of unity. In applications encountering a large dc offset on the cable or those wishing to ensure optimal rejection performance by avoiding differential offset error sources, dc blocking capacitors may be employed at the sense outputs of the SSM2142. As shown in the test circuit, these components should present as little impedance as possible to minimize low-frequency errors, such as 10 F NP (or tantalum if the polarity of the offset is known).
SYSTEM GROUNDING CONSIDERATIONS
100 90
100 90
10 0%
Figure 12. 100 kHz Square Wave Observed at Point B (Differential Mode). VO = 10 V rms, R1 = R2 = , RL = 600
10 0%
Due to ground currents, supply variations, and other factors, the ground potentials of the circuits at each end of a signal cable may not be exactly equal. The primary purpose of a balanced pair line is to reject this voltage difference, commonly called "longitudinal error." A measure of the ability of the system to reject longitudinal error voltage is output common-mode rejection. In order to obtain the optimal OCMR and noise rejection performance available with the SSM2142, the user should observe the following precautions: 1. The quality of the differential output is directly dependent upon the accuracy of the input voltage presented to the device. Input voltage errors developed across the impedance of the source must be avoided in order to maintain system performance. The input of the SSM2142 should be driven directly by an operational amplifier or buffer offering low source impedance and low noise. 2. The ground input should be in close proximity to the singleended input's source common. Ground offset errors encountered in the source circuitry also impair system performance.
5 6 1 V OUT
Figure 13. 100 kHz Square Wave at Point B (Differential Mode). VO = 10 V rms, R1 = R2 = , RL = 600 , with Series Feedback Capacitors
+15V +15V
V
4
6 7 SSM 2142 2 5 8 1 SHIELDED TWISTED-PAIR CABLE
7 3 2 SSM 2141/ 2143 4 -15V
IN
3. Make sure that the SSM2142 is adequately decoupled with 0.1 F bypass capacitors located close to each supply pin. 4. Avoid the use of passive circuitry in series with the SSM2142 outputs. Any reactive difference in the line pair will cause significant imbalances and affect the gain error of the device. Snubber networks or series load resistors are not required to maintain stability in SSM2142 based systems, even when driving signals over extremely long cables. 5. Efforts should be made to maintain a physical balance in the arrangement of the signal pair wiring. Capacitive differences due to variations in routing or wire length may cause unequal noise pickup between the pair, which will degrade the system OCMR. Shielded twisted-pair cable is the preferred choice in all applications. The shield should not be utilized as a signal conductor. Grounding the shield at one end, near the output common, avoids ground loop currents flowing in the shield which increase noise coupling and longitudinal errors.
3
-15V
Figure 14. Typical Application of the SSM2142 and SSM2141
APPLICATIONS INFORMATION
The SSM2142 is designed to provide excellent common-mode rejection, high output drive, and low signal distortion and noise in a balanced line-driving system. The differential output stage consists of twin cross-coupled unity gain buffer amplifiers with
REV. B
-5-
SSM2142
THE CABLE PAIR OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
The SSM2142 is capable of driving a 10 V rms signal into 600 and will remain stable despite cable capacitances of up to 0.16 F in either balanced or single-ended configurations. Low impedance shielded audio cable such as the standard Belden 8451 or similar is recommended, especially in applications traversing considerable distances. The user is cautioned that the so-called "audiophile" cables may incur four times the capacitance per unit length of the standard industrial-grade product. In situations of extreme load and/or distance, adding a second parallel cable allows the user to trade off half of the total line resistance against a doubling in capacitive load.
SINGLE-ENDED OPERATION
0.210 (5.33) MAX 0.160 (4.06) 0.115 (2.92)
8-Lead Plastic DIP
8 5 0.280 (7.11) 0.240 (6.10) 1 4 0.070 (1.77) 0.045 (1.15)
0.430 (10.92) 0.348 (8.84)
0.325 (8.25) 0.300 (7.62)
The SSM2142 is designed to be compatible with existing balanced-pair interface systems. Just as in transformer-based circuits, identical but opposite currents are generated by the output pair which can be ground-referenced if desired and transmitted on a single wire. Single-ended operation requires that the unused side of the output pair be grounded to a solid return path in order to avoid voltage offset errors at the nearby input common. The signal quality obtained in these systems is directly dependent on the quality of the ground at each end of the wire. Also note that in single-ended operation the gain through the device is still 6 dB, and that the SSM2142 incurs no significant degradation in signal distortion or output drive capability, although the noise rejection inherent in balancedpair systems is lost.
POWER SUPPLY SEQUENCING
0.015 (0.381) TYP
0.130 (3.30) MIN SEATING PLANE
0.015 (0.381) 0.008 (0.203)
0.022 (0.558) 0.014 (0.356)
0.100 (2.54) BSC
0 - 15
8-Lead Cerdip
0.005 (0.13) MIN 0.055 (1.35) MAX
8
5 0.310 (7.87) 0.220 (5.59)
1
4 0.070 (1.78) 0.030 (0.76)
A problem occasionally encountered in the interface system environment involves irregular application of the supplies. The user is cautioned that applying power erratically can inadvertently bias parts of the circuit into a latch-up condition. The small geometries of an integrated circuit are easily breached and damaged by short-risetime spikes on a supply line, which usually demonstrate considerable overshoot. The questionable practice of exchanging components or boards while under power can create such an undesirable sequence as well. Possible options which offer improved board-level device protection include: additional bypass capacitors, high-current reversebiased steering diodes between both supplies and ground, various transient surge suppression devices, and safety grounding connectors. Likewise, power should be applied to the device before the output is connected to "live" systems which may carry voltages of sufficient magnitude to turn on the output devices of the SSM2142 and damage the device. In any case, of course, the user must always observe the absolute maximum ratings shown in the specifications.
0.405 (10.29) MAX 0.200 (5.08) MAX 0.060 (1.52) 0.015 (0.38)
0.320 (8.13) 0.290 (7.37)
0.200 (5.08) 0.125 (3.18)
0.150 (3.81) MIN
0.015 (0.38) 0.008 (0.20)
0.023 (0.58) 0.014 (0.36)
0.100 (2.54) BSC
0 - 15 SEATING PLANE
16-Lead Small Outline (SOIC)
16 0.4193 (10.65) 0.3937 (10.00) 1
9 0 -8 0.2992 (7.60) 0.2914 (7.40) 8 0.0500 (1.27) 0.0157 (0.40) 0.0291 (0.74) 0.0098 (0.25) x 45
0.4133 (10.50) 0.3977 (10.10) 0.1043 (2.65) 0.0926 (2.35)
0.0118 (0.30) 0.0040 (0.10)
SEATING PLANE
0.0500 (1.27) BSC
0.0192 (0.49) 0.0138 (0.35)
0.0125 (0.32) 0.0091 (0.23)
SEE DETAIL ABOVE
-6-
REV. B
PRINTED IN U.S.A.
C1496-15-1/91

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