MSP430 datasheet(77/88 Pages) TI

77 / 88 page

The AFE88101 sets the DAC output voltage through an output code. This conversion to output voltage is set

through Equation 1; VMIN = 0.3 V and FSR = 2.2 V, resulting in Equation 14.

VOUT=DAC_CODE

216 ×2.2V+0.3V

(14)

In 4-mA to 20-mA systems, the nominal output operates from 4 mA as the low output and 20 mA as the high

output. However, systems sometimes use current outputs that are outside this range to indicate different error

conditions. Loop currents of 3.375 mA and 21.75 mA can be used to indicate different loop errors. Table 8-1

shows different loop output currents, along with the DAC code and voltages used.

Table 8-1. DAC Voltage Output and Loop Current Based on DAC Output Codes

OUTPUT CONDITION

DAC CODE

DAC OUTPUT (V)

LOOP CURRENT (mA)

DAC minimum

0x0000

0.3

Error low

0x045D

0.3375

3.375

In-range minimum

0x0BA2

0.4

In-range midscale

0x68BA

1.2

In-range maximum

0xC5D1

2.0

Error high

0xDA2E

2.175

21.75

DAC maximum

0xFFFF

2.5

Among the passive devices included in the design, choose gain setting resistors that exhibit tight tolerances to

achieve high accuracy. These resistors are primarily responsible for setting the gain of the current loop, along

with primary path of the output current flow.

8.2.1.2.3 Input Protection and Rectification

Figure 8-6 shows the simple protection scheme implemented in the design to mitigate issues that arise from

voltage and current transients on the bus. These transients have two main components: high-frequency and

high-energy. These two components can be leveraged with a strategy of attenuation and diversion by the

protection circuitry to deliver robust immunity.

TERMINAL

-V

TERMINAL

36-V

Bidirectional

TVS Diode

LOOP+

LOOP-

Ferrite

Bead

Figure 8-6. Loop Input Protection

Attenuation uses passive components, primarily resistors and capacitors, to attenuate high-frequency transients

and to limit series current. Use ferrite beads to maintain dc accuracy while still delivering the ability to limit

current from high-frequency transients. This circuit uses a capacitor placed across the input terminals, as well as

ferrite beads in series with the terminals.

Diversion capitalizes on the high-voltage properties of the transient signals by using a diode to clamp the

transient within supply voltages, or to divert the energy away from the system. Transient voltage suppressor

(TVS) diodes help protect against transients because TVS diodes break down very quickly and often feature

high power ratings that are critical to survive multiple transient strikes.

A rectifier is also implemented for reverse polarity protection so that the design can be connected to the bus

regardless of the pin orientation or polarity without damage to the design.

www.ti.com

AFE78101, AFE88101

SLASF21 – DECEMBER 2022

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