Advantech PCI-1720 Manual do Utilizador descarregar pdf (Página 31)

Chapter 5 Signal Connections 27

5.4 Current Sink Load and Power

Supply

You have to select the current sink load and power supply carefully.

The current sink circuitry of the PCI-1720 is as shown below.

Figure 5-7: PCI-1720 current sink circuitry

where

VS: Power supply voltage of current sink.

RL: Load of current sink.

PD: Power dissipation of FET.

When you determine VS and RL, three conditions must be satisfied.

VS > (RL + 249) ( 0.02)

VS ≤ 50

PD = (VS - (RL + 249) x 0.02) x 0.02 < 0.2

Example 1 If you use the internal power supply VS = +12 V,

RL must be less than 351 Ω (12 / 0.02 - 249 > RL). Select RL = 200

Ω, PD = 0.0604 < 0.2 OK!

Example 2 If you use the external power supply VS = +40 V,

RL must be less than 1.75 kΩ (40 / 0.02 - 249 > RL).

Select RL = 200 Ω, PD = 0.62 > 0.2 fail!

Reselect RL = 1 kΩ, PD = 0.3 > 0.2 fail!

Reselect RL = 1.5 kΩ, PD = 0.1 < 0.2 OK!

Internal Side External Side

AGND

0 ~ 5 V

1 ~ 5 V

249

Ω

0 ~ 20 mA

4 ~ 20 mA

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Advantech PCI-1720 Manual do Utilizador Página 31