8

LT1236

Kelvin Connections

Although the LT1236 does not have true force/sense

capability at its outputs, significant improvements in ground

loop and line loss problems can be achieved with proper

hook-up. In series mode operation, the ground pin of the

LT1236 carries only

≈ 1mA and can be used as a sense

line, greatly reducing ground loop and loss problems on

the low side of the reference. The high side supplies load

current so line resistance must be kept low. Twelve feet of

#22 gauge hook-up wire or 1 foot of 0.025 inch printed

circuit trace will create 2mV loss at 10mA output current.

This is equivalent to 1LSB in a 10V, 12-bit system.

The following circuits show proper hook-up to minimize

errors due to ground loops and line losses. Losses in the

output lead can be greatly reduced by adding a PNP boost

transistor if load currents are 5mA or higher. R2 can be

added to further reduce current in the output sense lead.

Effects of Air Movement on Low Frequency Noise

The LT1236 has very low noise because of the buried zener

used in its design. In the 0.1Hz to 10Hz band, peak-to-peak

noise is about 0.5ppm of the DC output. To achieve this

low noise, however, care must be taken to shield the

reference from ambient air turbulence. Air movement can

create noise because of thermoelectric differences be-

tween IC package leads and printed circuit board materials

and/or sockets. Power dissipation in the reference, even

though it rarely exceeds 20mW, is enough to cause small

Series Mode with Boost Transistor

LT1236

OUT

GND

IN

LOAD

R1

220

Ω

2N3906

R2*

INPUT

GROUND

RETURN

*OPTIONAL—REDUCES CURRENT IN OUTPUT SENSE

LEAD: R2 = 2.4k (LT1236-5), 5.6k (LT1236-10)

LT1236 AI04

temperature gradients in the package leads. Variations in

thermal resistance, caused by uneven air flow, create

differential lead temperatures, thereby causing thermo-

electric voltage noise at the output of the reference.

Standard Series Mode

LT1236

OUT

IN

GND

KEEP THIS LINE RESISTANCE LOW

LOAD

+

INPUT

GROUND

RETURN

LT1236 AI03

LT1236 TA10

LT1236A-10

OUT

IN

GND

TRIM

R2

50k

10.000V

R1

75k

TRIM RANGE

≈ ±10mV

Negative Series Reference

LT1236-10

OUT

IN

GND

LT1236 TA04

D1

15V

R1

4.7k

–10V AT

50mA

R2

4.7k

–15V

15V

Q1

2N2905

Restricted Trim Range for Improved

Resolution, 10V, “A” Version Only

LT1236 TA03

LT1236-10

OUT

IN

GND

TRIM

R1*

10k

CAN BE RAISED TO 20k FOR LESS

CRITICAL APPLICATIONS

*

LT1236-10 Full Trim Range (

±0.7%)