6

LT1031/LH0070

1031fb

TYPICAL PERFOR A CE CHARACTERISTICS

Output Noise 0.1Hz to 10Hz

TIME (MINUTES)

03

5

12

46

LT1031 • TPC16

10

µV (1ppm)

FILTERING = 1 ZERO AT 0.1Hz

2 POLES AT 10Hz

APPLICATIO S I FOR ATIO

Trimming Output Voltage

The LT1031 output can be trimmed by driving the ground

pin. The suggested method is shown in the illustration

below. A 5

Ω resistor is inserted in series with the ground

pin. The top of the resistor is supplied current from a trim

potentiometer. This technique requires fairly high trim

current of up to 1.5mA from the LT1031 or 3.5mA from the

–15V supply; however it is necessary to maintain low drift

in the reference. Ground pin current changes in the LT1031,

with temperature, could be as high as 4

µA/°C. This,

coupled with the 5

Ω external resistor, creates up

to 2ppm/

°C drift in the reference (5Ω • 4µA/°C = 20µV/°C

= 2ppm/

°C). If induced drift higher than this can be

tolerated, all resistor values in the trim circuit can be raised

proportionately to reduce current drain.

Output Voltage Trimming

LT1031 • TA04

–15V

*CAN BE INCREASED TO 5.6k FOR

LT1031B AND LH0070-2

**INCREASE TO 10

Ω FOR LT1031D

R3

50k

R2*

4.3k

R1**

5

Ω

LT1031

GND

IN

OUT

Effect of Reference Drift on System Accuracy

A large portion of the temperature drift error budget in

many systems is the system reference voltage. The graph

below indicates the maximum temperature coefficient

allowable if the reference is to contribute no more than

1/2LSB error to the overall system performance. The

example shown is a 12-bit system designed to operate

over a temperature range from 25

°C to 65°C. Assuming

the system calibration is performed at 25

°C, the

temperature span is 40

°C. The graph shows that the

temperature coefficient of the reference must be no worse

than 3ppm/

°C if it is to contribute less than 1/2LSB error.

For this reason, the LT1031 has been optimized for low

drift.

Maximum Allowable Reference Drift

TEMPERATURE SPAN (

°C)

10

30

100

1.0

10

20

100

90

80

70

60

50

40

LT1031 • TA03

14-BIT

12-BIT

10-BIT

8-BIT