Universal a LVDT Signal Conditioner AD698 FEATURES FUNCTIONAL BLOCK DIAGRAM Single Chip Solution, Contains Internal Oscillator and Voltage Reference No Adjustments Required VOLTAGE AMP REFERENCE Interfaces to Half-Bridge, 4-Wire LVDT OSCILLATOR DC Output Proportional to Position 20 Hz to 20 kHz Frequency Range AD698 B Unipolar or Bipolar Output Will Also Decode AC Bridge Signals A B Outstanding Performance AMP FILTER Linearity: 0.05% A Output Voltage: 611 V Gain Drift: 20 ppm/8C (typ) Offset Drift: 5 ppm/8C (typ) PRODUCT DESCRIPTION PRODUCT HIGHLIGHTS The AD698 is a complete, monolithic Linear Variable Differen- 1. The AD698 offers a single chip solution to LVDT signal tial Transformer (LVDT) signal conditioning subsystem. It is conditioning problems. All active circuits are on the mono- used in conjunction with LVDTs to convert transducer mechan- lithic chip with only passive components required to com- ical position to a unipolar or bipolar dc voltage with a high de- plete the conversion from mechanical position to dc voltage. gree of accuracy and repeatability. All circuit functions are 2. The AD698 can be used with many different types of posi- included on the chip. With the addition of a few external passive tion sensors. The circuit is optimized for use with any components to set frequency and gain, the AD698 converts the LVDT, including half-bridge and series opposed, (4 wire) raw LVDT output to a scaled dc signal. The device will operate configurations. The AD698 accommodates a wide range of with half-bridge LVDTs, LVDTs connected in the series op- input and output voltages and frequencies. posed configuration (4-wire), and RVDTs. 3. The 20 Hz to 20 kHz excitation frequency is determined by a The AD698 contains a low distortion sine wave oscillator to single external capacitor. The AD698 provides up to 24 volts drive the LVDT primary. Two synchronous demodulation rms to differentially drive the LVDT primary, and the channels of the AD698 are used to detect primary and second- AD698 meets its specifications with input levels as low as ary amplitude. The part divides the output of the secondary by 100 millivolts rms. the amplitude of the primary and multiplies by a scale factor. 4. Changes in oscillator amplitude with temperature will not af- This eliminates scale factor errors due to drift in the amplitude fect overall circuit performance. The AD698 computes the of the primary drive, improving temperature performance and ratio of the secondary voltage to the primary voltage to deter- stability. mine position and direction. No adjustments are required. The AD698 uses a unique ratiometric architecture to eliminate 5. Multiple LVDTs can be driven by a single AD698 either in several of the disadvantages associated with traditional ap- series or parallel as long as power dissipation limits are not proaches to LVDT interfacing. The benefits of this new cir- exceeded. The excitation output is thermally protected. cuit are: no adjustments are necessary temperature stability is improved and transducer interchangeability is improved. 6. The AD698 may be used as a loop integrator in the design of simple electromechanical servo loops. The AD698 is available in two performance grades: 7. The sum of the transducer secondary voltages do not need to Grade Temperature Range Package be constant. AD698AP 40C to +85C 28-Pin PLCC AD698SQ 55C to +125C 24-Pin Cerdip REV. B Information furnished by Analog Devices is believed to be accurate and Analog Devices, Inc., 1995 reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A. which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Tel: 617/329-4700 Fax: 617/326-8703( T = +258C, V = 0 V, and V+, V = 615 V dc, unless otherwise noted) AD698SPECIFICATIONS A CM AD698SQ AD698AP Parameter Min Typ Max Min Typ Max Unit 1 A V = 500 A R2 TRANSFER FUNCTION V OUT B OVERALL ERROR T to T 0.4 1.65 0.4 1.65 % of FS MIN MAX SIGNAL OUTPUT CHARACTERISTICS Output Voltage Range 611 611 V Output Current, T to T 11 11 mA MIN MAX Short Circuit Current 20 20 mA 2 Nonlinearity T to T 75 6500 75 6500 ppm of FS MIN MAX 3 Gain Error 0.1 61.0 0.1 61.0 % of FS Gain Drift 20 6100 20 6100 ppm/C of FS Output Offset 0.02 61 0.02 61 % of FS Offset Drift 5 625 5 625 ppm/C of FS 4 Excitation Voltage Rejection 100 100 ppm/dB Power Supply Rejection (12 V to 18 V) PSRR Gain 50 300 50 300 ppm/V PSRR Offset 15 100 15 100 ppm/V Common-Mode Rejection (3 V) CMRR Gain 25 100 25 100 ppm/V CMRR Offset 2 100 2 100 ppm/V 5 Output Ripple 4 4 mV rms EXCITATION OUTPUT CHARACTERISTICS ( 2.5 kHz) Excitation Voltage Range 2.1 24 2.1 24 V rms Excitation Voltage (Resistors Are 1% Absolute Values) 6 (R1 = Open) 1.2 2.15 1.2 2.15 V rms (R1 = 12.7 k) 2.6 4.35 2.6 4.35 V rms (R1 = 487 ) 14 21.2 14 21.2 V rms 7 Excitation Voltage TC 100 100 ppm/C Output Current 30 50 30 50 mA rms T to T 40 40 mA rms MIN MAX Short Circuit Current 60 60 mA DC Offset Voltage (Differential, R1 = 12.7 k) T to T 30 6100 30 6100 mV MIN MAX Frequency 20 20 k 20 20 k Hz Frequency TC 200 200 ppm/C Total Harmonic Distortion 50 50 dB SIGNAL INPUT CHARACTERISTICS A/B Ratio Usable Full-Scale Range 0.1 0.9 0.l 0.9 Signal Voltage B Channel 0.1 3.5 0.1 3.5 V rms Signal Voltage A Channel 0.0 3.5 0.0 3.5 V rms Input Impedance 200 200 k Input Bias Current (BIN, AIN) 1 5 1 5 A Signal Reference Bias Current 2 10 2 10 A Excitation Frequency 0 20 k 0 20 k Hz POWER SUPPLY REQUIREMENTS Operating Range 13 36 13 36 V Dual Supply Operation (10 V Output) 13 13 V Single Supply Operation 0 V to +10 V Output 17.5 17.5 V 0 V to 10 V Output 17.5 17.5 V Current (No Load at Signal and Excitation Outputs) 12 15 12 15 mA T to T 18 18 mA MIN MAX OPERATING TEMPERATURE RANGE 55 +125 40 +85 C 2 REV. B