AD5413 datasheet(27/49 Pages) AD

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Data Sheet

AD5413

Rev. 0 | Page 27 of 49

APPLICATIONS INFORMATION

VOLTAGE OUTPUT

Voltage Output Amplifier and +VSENSE Functionality

The voltage output amplifier can generate both unipolar and

bipolar output voltages. The amplifier can use an external

compensation capacitor connected to the AGND signal to drive

a 1 kΩ load in parallel with 2 μF.

Figure 60 shows the voltage output driving a load, RLOAD, on top

of a common-mode voltage (VCM) of ±10 V. An integrated 2 MΩ

resistor ensures that the amplifier loop is kept closed and

prevents potentially large and damaging voltages on VIOUT

because of the broken amplifier loop in applications where a

cable can disconnect from +VSENSE. If remote sensing of the load

is not required, connect +VSENSE directly to VIOUT via the 1 kΩ

series resistor and connect −VSENSE directly to AGND via the

external 1 kΩ series resistor.

VOUT

RANGE

SCALING

2MΩ

+VSENSE

–VSENSE

RLOAD

VIOUT

14-BIT

DAC

REFIN

AD5413

2MΩ

VCM

±10V

Figure 60. Voltage Output Load Connection

Driving Large Capacitive Loads

An additional external, 220 pF, nonpolarized compensation

capacitor allows the voltage output amplifier to drive capacitive

loads of up to 2 μF. This capacitor allows the AD5413 to drive

higher capacitive loads and reduce overshoot, but this capacitor

also increases the device settling time and negatively affects the

bandwidth of the system. Without the compensation capacitor,

capacitive loads of up to 10 nF can be driven.

Voltage Output Short-Circuit Protection

Under normal operation, the voltage output sinks and sources

up to 12 mA and maintains specified operation. The short-circuit

current is typically 15 mA. If a short circuit is detected, the

FAULT pin goes low and the VOUT_SC_ERR bit in the

ANALOG_DIAG_RESULTS register is set.

CURRENT OUTPUT

External Current Setting Resistor

As shown in Figure 57, RSET is an internal sense resistor that

forms part of the voltage to current conversion circuitry. The

stability of the output current value overtemperature depends

on the stability of the RSET value. To improve the output current

overtemperature stability, connect an external, 13.7 kΩ, low drift

resistor between the AD5413 RA pin and RB pin instead of the

internal resistor.

Table 1 shows the AD5413 performance specifications with both

the internal RSET resistor and an external 13.7 kΩ RSET resistor. The

external RSET resistor specification assumes an ideal resistor. The

actual device performance depends on the absolute value and

temperature coefficient of the resistor used. Therefore, the

resistor specifications directly affect the gain error of the output

and the TUE.

To arrive at the absolute worst case overall TUE of the output with

a particular external RSET resistor, add the percentage of the RSET

resistor absolute error (the absolute value of the error) to the TUE

of the AD5413 that is using the RSET external resistor shown in

Table 1 (expressed in % FSR). Consider the temperature coefficient

as well as the specifications of the external reference if this option is

used in the system.

The magnitude of the error (derived from summing the absolute

error and the temperature coefficient error of the external RSET

resistor and the external reference with the AD5413 TUE

specification) is unlikely to occur because the temperature

coefficient values of the individual components are unlikely to

exhibit the same drift polarity and an element of cancellation

occurs. For this reason, add the temperature coefficient values

with a root of squares method. To gain further TUE specification

improvement, perform a two-point calibration at zero-scale and

full-scale, which reduces the absolute errors of the voltage reference

and the RSET resistor.

Current Output Open-Circuit Detection

When in current output mode, if the available headroom falls

below the compliance range because of an open-loop circuit or

an insufficient power supply voltage, the IOUT_OC_ERR bit

flag in the ANALOG_DIAG_RESULTS register is asserted and

the FAULT pin goes low.

HART CONNECTIVITY

A HART signal can be ac-coupled onto the AD5413 CHART pin. The

HART signal appears on the current output if the HART_EN bit in

the GP_CONFIG1 register as well as the VIOUT output are enabled.

Figure 61 shows the recommended circuit for attenuating and

coupling the HART signal into the AD5413. To achieve 1 mA p-p

at the VIOUT pin, a signal of approximately 125 mV p-p is required

at the CHART pin. The HART signal on the VIOUT pin is inverted

relative to the signal input at the CHART pin.

In addition to being used to attenuate the incoming HART modem

signal, a minimum capacitance of the C1 and C2 capacitors is

required to ensure that the bandwidth presented to the modem

output signal allows the 1.2 kHz and 2.2 kHz frequencies through

the capacitors. Assuming a HART signal of 500 mV p-p, the

recommended capacitance values are C1 = 47 nF and C2 = 150 nF.

Digitally controlling the output slew rate is required to meet the

analog rate of change requirements for the HART protocol.

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