AN201 查看數據表（PDF） - Vishay Semiconductors

零件编号

产品描述 (功能)

比赛名单

AN201

High-Performance Multiplexing with the DG408

Vishay Semiconductors 

AN201

Vishay Siliconix

TC1

Instrumentation

TC2

Amplifier

+15 V

+

AV = 100

–

A/D

Converter

Output

–15 V

TC3

TC4

DG409

A0 A1

FIGURE 13. Differential Multiplexing of Thermocouples

The advantages of low-level or high-level transmission is

again dictated by system configuration and cost.

Preamplification at the transducer will provide low source

impedance and voltage gain, but it introduces the problem of

supplying power to the amplifier.

The transmission cable carrying the transducer signal is

critical in a low-level system; it should be as short as practical.

Signal conductors should be tightly twisted for minimum

enclosed area. This technique guards against picking up

electromagnetic interference and shields the twisted pair of

wires against capacitively-coupled (electrostatic)

interference. A key requirement for the transmission cable is

that it presents a balanced line to the source of noise

interference. This requires an equal series impedance in each

conductor and an equally distributed impedance from each

conductor to ground. The result should be that noise will be

coupled in phase to both conductors and rejected as

common-mode voltages. Coaxial cable is not suitable for

low-level signals because the two conductors (center and

shield) are unbalanced. Also ground loops are produced if the

shield is grounded at both ends by standard baby “N”

connector sockets. If coaxial cable is used, the signal should

be carried on the center conductors of two equal-length cables

whose shield is terminated only at the transducer end.

Document Number: 70600

05-Aug -99

Silicon in contact with aluminum creates a thermocouple

voltage. In a typical multiplexer, the source voltage will be

exactly canceled by the drain voltage, but large thermal

gradients between source and drain contacts can produce a

net offset voltage. The low current consumption of the DG409

("75 mA) translates into minimal errors due to thermocouple

offsets.

Programmable Amplifier

Figure 14 shows a programmable amplifier with selectable

inputs. This configuration is used in auto-ranging digital volt

meters, signal conditioners, etc. Its purpose is to select

optimum gain ready for conversion.

Gain ratios are a function of the resistor ratios. Sources of error

will be due to the on-resistance of the switches contributing to

the resistor ratio. The low on-resistance of the DG408

(typically 40 W at 25_C) and close matching should minimize

ranging errors introduced by the switches. The switching

speed of the DG408 will allow the preferred value of gain to be

attained quickly.

www.vishay.com

9

Share Link: