7

LT1031/LH0070

1031fb

APPLICATIO S I FOR ATIO

Standard Series Mode

LT1031 • TA05

INPUT

LOAD

+

KEEP THIS LINE RESISTANCE LOW

GROUND

RETURN

LT1031

GND

IN

OUT

Capacitive Loading and Transient Response

The LT1031 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

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 applications, a 10

µF solid tantalum

capacitor with several ohms in series provides optimum

output bypass.

Kelvin Connections

Although the LT1031 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

LT1031 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 LT1031 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

between IC package leads (especially kovar lead TO-5) and

printed circuit board materials and/or sockets. Power

dissipation in the reference, even though it rarely exceeds

20mW, is enough to cause small temperature gradients in

the package leads. Variations in thermal resistance, caused

by uneven airflow, create differential lead temperatures,

thereby causing thermoelectric voltage noise at the output

of the reference. The XY plotter trace shown on the

following page dramatically illustrates this effect. The first

half of the plot was done with the LT1031 shielded from

ambient air with a small foam cup. The cup was then

removed for the second half of the trace. Ambient in both

cases was a lab environment with no excessive air turbu-

lence from air conditioners, opening/closing doors, etc.

Removing the foam cup increases the output noise by

almost an order of magnitude in the 0.01Hz to 1Hz band!

The kovar leads of the TO-5 (H) package are the primary

culprit. Alloy 42 and copper lead frames used on dual-in-

line packages are not nearly as sensitive to thermally

generated noise because they are intrinsically matched.

Series Mode with Boost Transistor

LT1031

GND

OUT

IN

2N3906

INPUT

LOAD

*OPTIONAL—REDUCES CURRENT IN OUTPUT SENSE LEAD

GROUND

RETURN

R1

220

Ω

R2*

5.6k

LT1031 • TA06