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AD2S93AP Datasheet(PDF) 5 Page - Analog Devices |
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AD2S93AP Datasheet(HTML) 5 Page - Analog Devices |
5 / 12 page –5– REV. A AD2S93 GLOSSARY OF TERMS INTEGRAL LINEARITY Integral linearity deviation as a percent of full scale. A 0.1% de- viation is equivalent to 8-LSB change on the output. Gain The converter gain is the maximum variation in the ratio of A–B/REF/2 to the maximum digital input. Output Offset The output offset is the digital output code when the analog in- put signal A–B = 0. Overrange (OVR) OVR goes high when A–B is in phase with REF and larger than REF/2. Underrange (UNR) UNR goes high when A–B is out of phase with REF and larger than REF/2. PRINCIPLE OF OPERATION The AD2S93 is based on a Type 2 tracking closed-loop prin- ciple. The output tracks the position of the LVDT without the need for external convert and wait states. As the transducer moves through a position equivalent to the least significant bit weighting, the output is updated by one LSB. On the AD2S93, CLKOUT updates corresponding to one LSB increment. Fig- ure 1 illustrates the principle of operation. DEMODIN CLKOUT DEMOD OUT ERROR AMP LOS LATCHES FREQUENCY SHAPING VCO PHASE SENSITIVE DEMODULATOR C1 C2 R2 VEL UP-DOWN COUNTER REFERENCE (PRIMARY EXCITATION) DIFFERENTIAL (SECONDARY VOLTAGE) REF A B LOS GAIN V DD OVR UNR NULL CS DATA SCLK ACERROR AC RATIO BRIDGE DIFF DECODE LOGIC C3 AD2S93 INTIN R4 R3 SERIAL INTERFACE DIR R5 R1 C4 R6 R7 VCO GAIN Figure 1. Functional Block Diagram Because the conversion depends on the ratio of the input signals (ratiometric ac bridge), the AD2S93 is remarkably tolerant of input amplitude and frequency. This, combined with the defin- able Type 2 tracking closed-loop guarantees the AD2S93's re- peatability for a given input. A phase sensitive detector, integrator and voltage controlled oscillator (VCO) form a closed loop system which seeks to null the output of the ACERROR. When this is accomplished the word state of the up/down counter equals within the rated accuracy of the converter, the LVDT position output. For more information on the operation of the converter, see “Circuit Dynamics” section. DATA FORMAT OPERATING RANGE The AD2S93 operating range is defined in Figure 2. The lin- earity and specified operating range of the converter is the cen- tral two 12-bit quadrants through zero. The corresponding input relationship is –REF/2 ≤ A–B ≤ +REF/2, (± is used to de- note phase coherency). The sign bit is low for inputs with A–B in phase with REF. The two remaining 12-bit quadrants are used to denote over (OVR) and underrange (UNR). OVR goes high when A–B is in phase with REF and larger than REF/2. UNR goes high when A–B is out of phase with REF and larger than REF/2. LOS is an open drain output which pulls high when A and/or B are removed or REF is removed (see “Inbuilt Diagnostics”), or A + B is less than 100 mV. SCALING THE INPUTS In order to match the LVDT output to the AD2S93 output, the inputs to the AD2S93 need to be scaled. The operating range is illustrated in Figure 2. The AD2S93 operates across ±12-bit range where the remaining 12-bit quadrants are used to denote overrange and underrange. The output position word is a func- tion of the ratio between A–B and VREF (see Figure 2) where: ±FSR = ( A − B ) VREF /2 |
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