7
LT1031/LH0070
1031fb
APPLICATIO S I FOR ATIO
WUUU
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
worst-case occurs at I
LOAD
= 1.4mA (sinking).
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 10µF 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
220Ω
R2*
5.6k
LT1031 • TA06
