LTC1599C datasheet(13/20 Pages) LINER

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LTC1599

APPLICATIONS INFORMATION

or 10V full-scale systems. However, as the circuit band-

widths increase, filtering the output of the reference may

be required to minimize output noise.

Table 5. Partial List of LTC Precision References Recommended

for Use with the LTC1599, with Relevant Specifications

INITIAL

TEMPERATURE

0.1Hz to 10Hz

REFERENCE

TOLERANCE

DRIFT

NOISE

LT1019A-5,

±0.05%

5ppm

µVP-P

LT1019A-10

LT1236A-5,

±0.05%

5ppm

µVP-P

LT1236A-10

LT1460A-5,

±0.075%

10ppm

µVP-P

LT1460A-10

Grounding

As with any high resolution converter, clean grounding is

important. A low impedance analog ground plane and star

grounding should be used. IOUT2F and IOUT2S must be tied

to the star ground with as low a resistance as possible.

When it is not possible to locate star ground close to

IOUT2F and IOUT2S, separate traces should be used to route

these pins to star ground. This minimizes the voltage drop

from these pins to ground caused by the code dependent

current flowing to ground. When the resistance of these

circuit board traces becomes greater than 1

Ω, the circuit

in Figure 4 eliminates voltage drop errors caused by high

resistance traces. This preserves the excellent accuracy

(1LSB INL and DNL) of the LTC1599.

A 16-Bit, 4mA to 20mA Current Loop Controller

for Industrial Applications

Modern process control systems must often deal with

legacy 4mA to 20mA analog current loops as a means of

interfacing with actuators and valves located at a distance.

The circuit in Figure 5 provides an output to a current loop

controlled by an LTC1599, a 16-bit current output DAC. A

dual rail-to-rail op amp (U1, LT1366) controls a P-channel

power FET (Q2) to produce a current mirror with a precise

8:1 ratio as defined by a resistor array. The input current

to this mirror circuit is produced by a grounded base

cascode stage using a high gain transistor (Q1). The use

of a bipolar transistor in this location results in an error

term associated with U1B and Q1’s base current (– 0.2%

for the device shown). For control applications however,

absolute accuracy of the output to an actuator is usually

not required. If a higher degree of absolute accuracy is

required, Q1 can be replaced with an N-channel JFET;

however, this requires a single amplifier at U1B with the

ability to drive the gate below ground. An enhancement

mode N-channel FET can be used in place of Q1 but

MOSFET leakage current must be considered and gate

overdrive must be avoided.

Figure 4. Driving IOUT2F and IOUT2S with a Force/Sense Amplifier

VCC

LTC1599

RFB

ROFS

RCOM

REF

0.1

µF

IOUT1

33pF

VOUT

0V TO –10V

1599 F04

LT1001

–

LT1001

16-BIT DAC

IOUT2F

IOUT2S

MLBYTE

14 TO 18,

21 TO 23

DATA

INPUTS

CLR

CLVL

CLR

DGND

CLVL

LT1236A-10

6 10V

15V

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