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SSM2018TS2 Datasheet(PDF) 10 Page - Analog Devices |
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SSM2018TS2 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page REV. B –10– SSM2018T Proper Operating Mode for the SSM2018T The SSM2018T has the flexibility of operating in either Class A or Class AB. This is accomplished by adjusting the amount of current flowing in the gain core (IM in Figure 2). The traditional trade-off between the two classes is that Class A tends to have lower THD but higher noise than Class AB. However, by using well matched gain core transistors, distortion compensation circuitry and laser trimming, the SSM2018T has excellent THD performance in Class AB. Thus, it offers the best of both worlds in having the low noise of Class AB with low THD. Because the SSM2018T operates optimally in Class AB, the distortion trim is performed for this class. To guarantee conform- ance to the data sheet THD specifications, the SSM2018T must be operated in class AB. This does not mean that it can not be oper- ated in Class A, but the optimal THD trim point is different for the two classes. Using Class A operation results to 0.05% with- out trim. An external potentiometer could be added to change the trim back to its optimal point as shown in the OVCE appli- cation circuit, but this adds the expense and time in adjusting a potentiometer. The class of operation is set by selecting the proper value for RB shown in Figure 1. RB determines the current flowing into the MODE input (Pin 12). For class AB operation with ± 15 V supplies, RB should be 150 k W. This results in a current of 95 mA. For other supply voltages, adjust the value of R B such that current remains at 95 mA. This current follows the formula: I MODE = (V CC –0. 7 V ) R B (3) 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 W load. Note: This 100 k W load is in parallel with the feedback resistor of 18 kW, so the effective load is 15.3 k W. For better than 0.01% distortion, the output should remain about 3.5 V away from either rail as shown in TPC 2. As the graph of output swing versus load resis- tance shows (TPC 9), to maintain less than 1% distortion the output current should be limited to approximately ±1.3 mA. If higher current drive is required, 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 W series resistor. 18k 50pF V+ VOUT 47pF NC 1 F 1k 3k V– V+ 1 F 18k 1 F 18k RB: 150k FOR CLASS AB NC = NO CONNECT RB VCONTROL VIN+ VIN– SSM2018T 470k 500k 100k 10M OFFSET TRIM V+ V– SYMMETRY TRIM REMOVE FOR SSM2018T Figure 3. Upgrading SSM2018 Sockets Upgrading SSM2018 Sockets The SSM2018T easily replaces the SSM2018 in the basic VCA configuration. The parts are pin for pin compatible allowing direct replacement. At the same time, the trimming potentiom- eters for symmetry and offset should be removed, as shown in Figure 3. Upgrading immediately to the SSM2018T 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 potentiom- eters 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 temperature 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 ±20 dB. If the application requires the gain constant to be the same over a wide temperature range, external temperature com- pensation should be employed. The simplest form of compensa- tion is a temperature compensating resistor (TCR) such as the PT146 from Precision Resistor Co. These elements are different than a standard thermistor in that they are linear over tempera- ture to better match the linear drift of the gain constant. *PT146 AVAILABLE FROM PRECISION RESISTOR CO. 10601 75TH ST. NORTH LARGO, FL 34647 (813) 541-5771 +15V –15V 1k * 2k OP27 PIN 11 SSM2018T CONTROL VOLTAGE Figure 4. Two TCRs Compensate for Temperature Drift of Gain Constant |
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