LT1236LS8 datasheet(7/16 Pages) LINER

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LT1236LS8

1236ls8f

applicaTions inForMaTion

Effect of Reference Drift on System Accuracy

A large portion of the temperature drift error budget in

many systems is the system reference voltage. This graph

indicates the maximum temperature coefficient allowable

if the reference is to contribute no more than 0.5LSB error

to the overall system performance. The example shown is

a 12-bit system designed to operate over a temperature

rangefrom25°Cto65°C.Assumingthesystemcalibration

is performed at 25°C, the temperature span is 40°C. It can

be seen from the graph that the temperature coefficient

of the reference must be no worse than 3ppm/°C if it is

to contribute less than 0.5LSB error. For this reason, the

LT1236LS8 has been optimized for low drift.

Trimming Output Voltage

The LT1236LS8 has an output voltage trim pin, but the

temperature drift of the nominal 4V open circuit voltage

at pin 5 is about –1.7mV/°C. For the voltage trimming not

to affect reference output temperature drift, the external

trim voltage must track the voltage on the trim pin. Input

impedance of the trim pin is about 100kΩ and attenua-

tion to the output is 13:1. The technique shown below

is suggested for trimming the output of the LT1236LS8

while maintaining minimum shift in output temperature

coefficient. The R1/R2 ratio is chosen to minimize interac-

tion of trimming and temperature drift shifts, so the exact

values shown should be used.

Capacitive Loading and Transient Response

The LT1236LS8 is stable with all capacitive loads, but for

optimum settling with load transients, output capacitance

should be under 1000pF. The output stage of the reference

is class AB with a fairly low idling current. This makes

transient response worst-case at light load currents.

Because of internal current drain on the output, actual

worst-case occurs at ILOAD = 0. Significantly better load

transient response is obtained by moving slightly away

from these points. See Load Transient Response curves

for details. In general, best transient response is obtained

when the output is sourcing current. In critical applica-

tions, a 10µF solid tantalum capacitor with several ohms

in series provides optimum output bypass.

Load Regulation

The LT1236LS8 is capable of driving 10mA to a load. The

load regulation at the output of the LT1236LS8 is very

good, with a change of less than 25ppm/mA when driving

the load. However, the load current will cause a voltage

drop in the connecting wire between the LT1236LS8 and

the load. This IR drop is dependent on the resistance of

the connecting wire and will appear as additional load

regulation error. For example, 12 feet of #22 gauge wire or

1 foot of 0.025 inch printed circuit board trace will create

2mV loss at 10mA output current. This is equivalent to

1LSB in a 10V, 12-bit system.

Therearethreeapproachesthatwillreducethiseffect.First,

limiting the distance between the LT1236LS8 and the load

will reduce the trace length, and improve load regulation.

Second, use wider traces for the connections between

the LT1236LS8 and the load to reduce IR drop. Finally,

TEMPERATURE SPAN (°C)

100

1236ls8 AI01

1.0

100

8-BIT

10-BIT

12-BIT

14-BIT

LT1236LS8

OUT

GND TRIM

27k

50k

1N4148

VOUT

1236ls8 AI02

Maximum Allowable Reference Drift

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