AD698 查看數據表（PDF） - Analog Devices

零件编号

产品描述 (功能)

生产厂家

AD698

Universal LVDT Signal Conditioner

Analog Devices 

AD698

Determining LVDT Sensitivity

LVDT sensitivity can be determined by measuring the LVDT

secondary voltages as a function of primary drive and core posi-

tion, and performing a simple computation.

Energize the LVDT at its recommended primary drive level,

VPRI (3 V rms for the E100). Set the core displacement to its

mechanical full-scale position and measure secondary voltages

VA and VB.

Sensitivity

=

VSECONDARY

VPRI × d

From Figure 19,

Sensitivity =

0. 72

= 2.4 mV /V mil

3 V × 100 mils

VSEC WHEN VPRI 3V rms

VA

1.71V rms

d = –100 mils d = 0

0.99V rms

VB

d = +100 mils

Figure 19. LVDT Secondary Voltage vs. Core

Displacement

Thermal Shutdown and Loading Considerations

The AD698 is protected by a thermal overload circuit. If the die

temperature reaches 165°C, the sine wave excitation amplitude

gradually reduces, thereby lowering the internal power dissipa-

tion and temperature.

Due to the ratiometric operation of the decoder circuit, only

small errors result from the reduction of the excitation ampli-

tude. Under these conditions the signal-processing section of

the AD698 continues to meet its output specifications.

The thermal load depends upon the voltage and current deliv-

ered to the load as well as the power supply potentials. An

LVDT Primary will present an inductive load to the sine wave

excitation. The phase angle between the excitation voltage and

current must also be considered, further complicating thermal

calculations.

APPLICATIONS

Most of the applications for the AD598 can also be imple-

mented with the AD698. Please refer to the applications written

for the AD598 for a detailed explanation.

See AD598 data sheet for:

– Proving Ring-Weigh Scale

– Synchronous Operation of Multiple LVDTs

– High Resolution Position-to-Frequency Circuit

– Low Cost Setpoint Controller

– Mechanical Follower Servo Loop

– Differential Gaging and Precision Differential Gaging

AC BRIDGE SIGNAL CONDITIONER

Bridge circuits which use dc excitation are often plagued by er-

rors caused by thermocouple effects, 1/f noise, dc drifts in the

electronics, and line noise pickup. One way to get around these

problems is to excite the bridge with an ac waveform, amplify

the bridge output with an ac amplifier, and synchronously de-

modulate the resulting signal. The ac phase and amplitude in-

formation from the bridge is recovered as a dc signal at the

output of the synchronous demodulator. The low frequency

system noise, dc drifts, and demodulator noise all get mixed to

the carrier frequency and can be removed by means of a low-

pass filter.

The AD698 with the addition of a simple ac gain stage can be

used to implement an ac bridge. Figure 20 shows the connec-

tions for such a system. The AD698 oscillator provides ac

excitation for the bridge. The low level bridge signal is amplified

by the gain stage created by A1, A2 to provide a differential in-

put to the A Channel of the AD698. The signal is then synchro-

nously detected by A Channel. The B Channel is used to detect

the level of the bridge excitation. The ratio of A/B is then calcu-

lated and converted to an output voltage by R2. An optional

phase lag/lead network can be added in front of the A compara-

tor to adjust for phase delays through the bridge and the ampli-

fier, or if the phase delay is small, it can be ignored or compensated

for by a gain adjustment.

This circuit can be used for resistive bridges such as strain

gages, or for inductive or capacitive bridges that are commonly

used for pressure or flow sensors. The low level signal outputs of

these sensors are susceptible to noise and interference and are

good candidates for ac signal processing techniques.

Component Selection

Amplifiers A1, A2 will be chosen depending on the type of

bridge that is conditioned. Capacitive bridges should use an

amplifier with low bias current; a large bleeder resistor will be

required from the amplifier inputs to ground to provide a path

for the dc bias current. Resistive and inductive bridges can use a

more general purpose amplifier. The dc performance of A1, A2

are not as important as their ac performance. DC errors such as

voltage offset will be chopped out by the AD698 since they are

not synchronous to the carrier frequency.

The oscillator amplitude and span resistor for the AD698 may

be chosen by first computing the transfer function or sensitivity

of the bridge and the ac amplifier. This ratio will correspond to

the A/B term in the AD698 transfer function. For example, sup-

pose that a resistive strain gage with a sensitivity, S, of 2 mV/V

at full scale is used. Select an arbitrary target value for A/B that

is close to its maximum value such as A/B = 0.8. Then choose a

gain for the ac amplifier so that the strain gage transfer function

from excitation to output also equals 0.8. Thus the required am-

plifier gain will be [A/B]/ S; or 0.8/ 0.002 V/V = 400. Then

select values for RS and RG. For the gain stage:

–10–

REV. B

Share Link: