LT1021DCN8-10 Datasheet, PDF (10/16 Pages) Linear Dimensions Semiconductor – Precision Reference

LT1021
APPLICATIONS INFORMATION
minimized. In the current output circuit, R1 determines
output current. It should have a TC commensurate with the
LT1021 or track closely with the feedback resistor around
the op amp.
LT1021-7
IN
OUT
GND
R3
10k
R2* ^R1*
14k ^200Ω
1%
1%
V_OUT
6.950V
TC TRACKING TO 50ppm/°C
1021 AI03
LT1021-7
IN
OUT
GND
R3
50k
R2**
182k
R1*
7.15k
1.000mA
Kelvin Connections
Although the LT1021 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
LT1021 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.
Standard Series Mode
–
OP AMP
+
1021 AI04
*RESISTOR TC DETERMINES I_OUT TC
**TC ≤ (10 • R1) TC. R2 AND R3 SCALE
WITH R1 FOR DIFFERENT OUTPUT CURRENTS
Capacitive Loading and Transient Response
The LT1021 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. Be-
cause of internal current drain on the output, actual worst-
case occurs at I_LOAD = 0 on LT1021-5, I_LOAD =
– 0.8mA (sinking) on LT1021-7 and I_LOAD = 1.4mA (sink-
ing) on LT1021-10. 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.
10
INPUT
LT1021
IN
OUT
GND
KEEP THIS LINE RESISTANCE LOW
+
LOAD
GROUND
RETURN
1021 AI05
Series Mode with Boost Transistor
INPUT
R1
220Ω
IN
LT1021
OUT
GND
2N3906
LOAD
R2*
GROUND
RETURN
1021 AI06
*OPTIONAL—REDUCES CURRENT IN OUTPUT SENSE LEAD
R2 = 2.4k (LT1021-5), 3k (LT1021-7), 5.6k (LT1021-10)
1021fc