Datasheet SA571 (ON Semiconductor) - 5
| Manufacturer | ON Semiconductor |
| Description | Compandor |
| Pages / Page | 12 / 5 — SA571. INTRODUCTION. Circuit Background. Basic Hook−up and Operation. … |
| File Format / Size | PDF / 230 Kb |
| Document Language | English |
SA571. INTRODUCTION. Circuit Background. Basic Hook−up and Operation. Figure 4. Restricted Dynamic Range Channel
Model Line for this Datasheet
Text Version of Document
link to page 5 link to page 5
SA571 INTRODUCTION
requires a simple full−wave averaging rectifier with good Much interest has been expressed in high performance accuracy, since the rectifier accuracy determines the (input) electronic gain control circuits. For non−critical output level tracking accuracy. The gain cell determines the applications, an integrated circuit operational distortion and noise characteristics, and the phone system transconductance amplifier can be used, but when specifications here are very loose. These specs could have high−performance is required, one has to resort to complex been met with a simple Operational Transconductance discrete circuitry with many expensive, well−matched Multiplier, or OTA, but the gain of an OTA is proportional components. This paper describes an inexpensive integrated to temperature and this is very undesirable. Therefore, a circuit, the SA571 Compandor, which offers a pair of high linearized transconductance multiplier was designed which performance gain control circuits featuring low distortion is insensitive to temperature and offers low noise and low (<0.1%), high signal−to−noise ratio (90 dB), and wide distortion performance. These features make the circuit dynamic range (110 dB). useful in audio and data systems as well as in telecommunications systems.
Circuit Background
The SA571 Compandor was originally designed to satisfy
Basic Hook−up and Operation
the requirements of the telephone system. When several Figure 5 shows the block diagram of one half of the chip, telephone channels are multiplexed onto a common line, the (there are two identical channels on the IC). The full−wave resulting signal−to−noise ratio is poor and companding is averaging rectifier provides a gain control current, IG, for the used to allow a wider dynamic range to be passed through variable gain (DG) cell. The output of the DG cell is a current the channel. Figure 4 graphically shows what a compandor which is fed to the summing node of the operational can do for the signal−to−noise ratio of a restricted dynamic amplifier. Resistors are provided to establish circuit gain and range channel. The input level range of +20 to −80 dB is set the output DC bias. shown undergoing a 2−to−1 compression where a 2.0 dB The circuit is intended for use in single power supply input level change is compressed into a 1.0 dB output level systems, so the internal summing nodes must be biased at change by the compressor. The original 100 dB of dynamic some voltage above ground. An internal band gap voltage range is thus compressed to a 50 dB range for transmission reference provides a very stable, low noise 1.8 V reference through a restricted dynamic range channel. A denoted VREF. The non−inverting input of the op amp is tied complementary expansion on the receiving end restores the to VREF, and the summing nodes of the rectifier and DG cell original signal levels and reduces the channel noise by as (located at the right of R1 and R2) have the same potential. much as 45 dB. The THD trim pin is also at the VREF potential. The significant circuits in a compressor or expander are the rectifier and the gain control element. The phone system THD TRIM R INV 3 IN ANSION 8,9 5,12 INPUT OUTPUT R 6,11 LEVEL LEVEL 3 COMPRESSION EXP G +20 −20 IN R 20kW 2 OUTPUT DG − 0dB 0dB 3,14 20kW R VREF 4 IG + 7,10 RECTIN R 30kW 1 1.8V −40 −40 V NOISE CC PIN 13 10kW 2,15 1,16 GND PIN 4 −80 −80 CRECT
Figure 4. Restricted Dynamic Range Channel Figure 5. Chip Block Diagram (1 of 2 Channels) http://onsemi.com 5
SA571 INTRODUCTION
requires a simple full−wave averaging rectifier with good Much interest has been expressed in high performance accuracy, since the rectifier accuracy determines the (input) electronic gain control circuits. For non−critical output level tracking accuracy. The gain cell determines the applications, an integrated circuit operational distortion and noise characteristics, and the phone system transconductance amplifier can be used, but when specifications here are very loose. These specs could have high−performance is required, one has to resort to complex been met with a simple Operational Transconductance discrete circuitry with many expensive, well−matched Multiplier, or OTA, but the gain of an OTA is proportional components. This paper describes an inexpensive integrated to temperature and this is very undesirable. Therefore, a circuit, the SA571 Compandor, which offers a pair of high linearized transconductance multiplier was designed which performance gain control circuits featuring low distortion is insensitive to temperature and offers low noise and low (<0.1%), high signal−to−noise ratio (90 dB), and wide distortion performance. These features make the circuit dynamic range (110 dB). useful in audio and data systems as well as in telecommunications systems.
Circuit Background
The SA571 Compandor was originally designed to satisfy
Basic Hook−up and Operation
the requirements of the telephone system. When several Figure 5 shows the block diagram of one half of the chip, telephone channels are multiplexed onto a common line, the (there are two identical channels on the IC). The full−wave resulting signal−to−noise ratio is poor and companding is averaging rectifier provides a gain control current, IG, for the used to allow a wider dynamic range to be passed through variable gain (DG) cell. The output of the DG cell is a current the channel. Figure 4 graphically shows what a compandor which is fed to the summing node of the operational can do for the signal−to−noise ratio of a restricted dynamic amplifier. Resistors are provided to establish circuit gain and range channel. The input level range of +20 to −80 dB is set the output DC bias. shown undergoing a 2−to−1 compression where a 2.0 dB The circuit is intended for use in single power supply input level change is compressed into a 1.0 dB output level systems, so the internal summing nodes must be biased at change by the compressor. The original 100 dB of dynamic some voltage above ground. An internal band gap voltage range is thus compressed to a 50 dB range for transmission reference provides a very stable, low noise 1.8 V reference through a restricted dynamic range channel. A denoted VREF. The non−inverting input of the op amp is tied complementary expansion on the receiving end restores the to VREF, and the summing nodes of the rectifier and DG cell original signal levels and reduces the channel noise by as (located at the right of R1 and R2) have the same potential. much as 45 dB. The THD trim pin is also at the VREF potential. The significant circuits in a compressor or expander are the rectifier and the gain control element. The phone system THD TRIM R INV 3 IN ANSION 8,9 5,12 INPUT OUTPUT R 6,11 LEVEL LEVEL 3 COMPRESSION EXP G +20 −20 IN R 20kW 2 OUTPUT DG − 0dB 0dB 3,14 20kW R VREF 4 IG + 7,10 RECTIN R 30kW 1 1.8V −40 −40 V NOISE CC PIN 13 10kW 2,15 1,16 GND PIN 4 −80 −80 CRECT
Figure 4. Restricted Dynamic Range Channel Figure 5. Chip Block Diagram (1 of 2 Channels) http://onsemi.com 5
