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AD588KQ Scheda tecnica(PDF) 4 Page - Analog Devices |
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AD588KQ Scheda tecnica(HTML) 4 Page - Analog Devices |
4 / 12 page REV. D –4– AD588 THEORY OF OPERATION The AD588 consists of a buried Zener diode reference, amplifiers used to provide pin programmable output ranges, and associ- ated thin-film resistors as shown in Figure 1. The temperature compensation circuitry provides the device with a temperature coefficient of 1.5 ppm/ °C or less. R3 RB R1 R2 R4 R5 R6 GAIN ADJ GND SENSE +IN GND SENSE –IN VLOW BAL ADJ VCT A4 IN –VS +VS A4 OUT FORCE A4 OUT SENSE A3 OUT FORCE A3 OUT SENSE A3 IN VHIGH NOISE REDUCTION A1 A4 AD588 A3 A2 Figure 1. AD588 Functional Block Diagram Amplifier A1 performs several functions. A1 primarily acts to amplify the Zener voltage from 6.5 V to the required 10 V output. In addition, A1 also provides for external adjustment of the 10 V output through Pin 5, GAIN ADJ. Using the bias compen- sation resistor between the Zener output and the noninverting input to A1, a capacitor can be added at the NOISE REDUCTION pin (Pin 7) to form a low-pass filter and reduce the noise contri- bution of the Zener to the circuit. Two matched 10 k Ω nominal thin-film resistors (R4 and R5) divide the 10 V output in half. Pin VCT (Pin 11) provides access to the center of the voltage span and Pin 12 (BAL ADJ) can be used for fine adjustment of this division. Ground sensing for the circuit is provided by amplifier A2. The noninverting input (Pin 9) senses the system ground, which will be transferred to the point on the circuit where the invert- ing input (Pin 10) is connected. This may be Pin 6, 8, or 11. The output of A2 drives Pin 8 to the appropriate voltage. Thus, if Pin 10 is connected to Pin 8, the VLOW pin will be the same voltage as the system ground. Alternatively, if Pin 10 is con- nected to the VCT pin, it will be ground and Pin 6 and Pin 8 will be +5 V and –5 V, respectively. Amplifiers A3 and A4 are internally compensated and are used to buffer the voltages at Pins 6, 8, and 11, as well as to provide a full Kelvin output. Thus, the AD588 has a full Kelvin capability by providing the means to sense a system ground and provide forced and sensed outputs referenced to that ground. APPLYING THE AD588 The AD588 can be configured to provide +10 V and –10 V reference outputs as shown in Figures 2a and 2c, respectively. It can also be used to provide +5 V, –5 V, or a ± 5 V tracking reference, as shown in Figure 2b. Table I details the appropriate pin connections for each output range. In each case, Pin 9 is connected to system ground and power is applied to Pins 2 and 16. The architecture of the AD588 provides ground sense and uncommitted output buffer amplifiers that offer the user a great deal of functional flexibility. The AD588 is specified and tested in the configurations shown in Figure 2a. The user may choose to take advantage of the many other configuration options available with the AD588. However, performance in these configurations is not guaranteed to meet the extremely stringent data sheet specifications. As indicated in Table I, a +5 V buffered output can be provided using amplifier A4 in the +10 V configuration (Figure 2a). A –5 V buffered output can be provided using amplifier A3 in the –10 V configuration (Figure 2c). Specifications are not guaranteed for the +5 V or –5 V outputs in these configurations. Performance will be similar to that specified for the +10 V or –10 V outputs. As indicated in Table I, unbuffered outputs are available at Pins 6, 8, and 11. Loading of these unbuffered outputs will impair circuit performance. Amplifiers A3 and A4 can be used interchangeably. However, the AD588 is tested (and the specifications are guaranteed) with the amplifiers connected as indicated in Figure 2a and Table I. When either A3 or A4 is unused, its output force and sense pins should be connected and the input tied to ground. Two outputs of the same voltage may be obtained by connect- ing both A3 and A4 to the appropriate unbuffered output on Pins 6, 8, or 11. Performance in these dual-output configura- tions will typically meet data sheet specifications. CALIBRATION Generally, the AD588 will meet the requirements of a precision system without additional adjustment. Initial output voltage error of 1 mV and output noise specs of 10 µV p-p allow for accuracies of 12 bits to 16 bits. However, in applications where an even greater level of accuracy is required, additional calibra- tion may be called for. Provision for trimming has been made through the use of the GAIN ADJ and BAL ADJ pins (Pins 5 and 12, respectively). The AD588 provides a precision 10 V span with a center tap (VCT) that is used with the buffer and ground sense amplifiers to achieve the voltage output configurations in Table I. GAIN ADJUST and BALANCE ADJUST can be used in any of these configurations to trim the magnitude of the span voltage and the position of the center tap within the span. The GAIN ADJUST should be performed first. Although the trims are not interactive within the device, the GAIN trim will move the BALANCE trim point as it changes the magnitude of the span. |
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