CN-0371

Circuit Note

Figure 3. Position Linearity Error vs. LVDT Core Displacement

The measured data shows that the circuit performance is better

than the linearity specified in the E-Series LVDT data sheet.

Synchronous Operation of Multiple LVDTs

In many applications, multiple LVDTs are used in close proximity.

If the LVDTs operate at similar carrier frequencies, stray magnetic

coupling can cause generation of beat notes. The resulting beat

notes can interfere with the accuracy of measurements made

under these conditions. To avoid this situation, all LVDTs must

be synchronized.

Multiple ADA2200 devices can be synchronized by bringing the

devices out of reset simultaneously. The ADA2200 leaves reset

mode on the first rising edge of CLKIN after the RST pin is

deasserted. Therefore, driving all of the ADA2200 CLKIN pins

from a single source and all of the RESETB lines from a single

source is sufficient to ensure the devices operate synchronously.

Avoid deasserting RESETB near the rising edge of CLKIN to

prevent the device resets from occurring on different clock

edges. The RCLK outputs of the ADA2200 devices can be

monitored to ensure that the ADA2200 devices are properly

synchronized.

COMMON VARIATIONS

In some systems, it may be desirable to use a SAR ADC instead

of a Σ-Δ converter. In these cases, the ADC conversion clock

must be synchronized with the ADA2200 update rate. As

described previously, the ADA2200 output will contain spurs

at multiples of the excitation frequency. These spurs can be

rejected with a moving average filter or by cascading multiple

moving average filters. Moving average filters are easy to

implement and exhibit excellent time-domain characteristics.

To completely reject the spurs, the moving average sample size

must be an integer multiple of 8 samples.

Figure 4 shows the ADA2200 connected to the AD7091R-2, an

ultralow power, dual channel, 12-bit SAR ADC. A dual channel

ADC is used so that the OUTP and OUTN outputs of the

ADA2200 can be sampled sequentially. Because the ADA2200

outputs are discrete time samples, differential measurements can

be made by performing the difference operation, VOUTP − VOUTN,

on the two sequential output samples.

Figure 4. Differential Sampling with Dual Channel SAR ADC

The SYNCO signal goes active each ADA2200 output sample

period. SYNCO can be used to interrupt the microcontroller

to take a pair of ADC samples (to sample OUTP and OUTN).

In the case demonstrated, the sample period is at 8 times the

excitation clock frequency or 38.4 kHz; therefore, the ADC

samples at a rate of 76.8 kSPS.

CIRCUIT EVALUATION AND TEST

The CN-0371 circuit uses the EVAL-CN0371-SDPZ circuit

board and the EVAL-SDP-CB1Z SDP-B controller board. The

two boards have 120-pin mating connectors, allowing quick

setup and evaluation of the performance of the circuit. The

circuit board contains the circuit to be evaluated, and the SDP-B

board is used with the CN-0371 Evaluation Software to capture

the data from the circuit board.

Equipment Needed

The following equipment is needed:

•

A PC with a USB port and Windows® XP (32-bit),

Windows Vista®, or Windows 7

•

The EVAL-CN0371-SDPZ circuit board

•

The EVAL-SDP-CB1Z SDP-B controller board

•

The CN-0371 Evaluation Software

•

Measurement Specialties, Inc., E-100 Economy Series LVDT

(EVAL-CFTL-LVDT)

Getting Started

Download the CN0371.zip file and run the setup.exe file. By

default, the program is installed in the Analog Devices directory.

The program name is CN0371.

Functional Block Diagram

See Figure 5 for the circuit block diagram. For the complete circuit

schematic, see the EVAL-CN0371-SDPZ-Schematic.pdf file,

which can be found in the CN-0371 Design Support Package.

The 5 V supply for the circuit board is supplied by the USB bus

via the SDP-B board 120-pin connector.

Figure 5. Test Setup Block Diagram

0.10

0.05

0

–0.05

DISPLACEMENT (mm)

–0.10

–0.15

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

–0.20

OUTP

MICROCONTROLLER

2:1

MUX

12-BIT

ADC

ADA2200

AD7091R-2

OUTN

SYNCO

CLKIN

IRQ

CLK0

CONVST

SEQUENCER

SDI

CS

SDO

SCLK

P0.1

MOSI

CS

MISO

SCLK

MEASUREMENT

SPECIALTIES, INC.

E-100 SERIES LVDT

EVAL-CFTL-LVDT

P1

P3

CON A

USB

PC

EVAL-CN0371-SDPZ

BOARD

EVAL-SDP-CB1Z

SDP-B BOARD

120

PINS

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