Aug 1998 New 16-Bit Bipolar Output DAC in Narrow SO-16 Package

DESIGN INFORMATION
switched output. The monolithic diode
bridge is also temperature controlled,
providing a bridge offset error below
10µ V, stabilizing the measurement
baseline. The temperature control is
implemented using uncommitted
diodes in the monolithic array as heater
and sensor.
Figure 5 details considerations for
the diode bridge switch. The bridge
diodes tend to cancel each other’s temperature coefficient—unstabilized
bridge drift is about 100µV/°C and the
temperature control reduces residual
drift to a few microvolts/°C.
Bridge temperature control is
achieved by using one diode as a sensor. Another diode, running in reverse
breakdown (V Z ≈ 7V), serves as the
heater. The control amplifier, comparing the sensor diode to a voltage at its
negative terminal, drives the heater
diode to temperature stabilize the array.
DC balance is achieved by trimming
the bridge on-current for zero input–
output offset voltage. Two AC trims are
required. The “AC balance” corrects for
diode and layout capacitive imbalances
and the “skew compensation” corrects
for any timing asymmetry in the nomi-
nally complementary bridge drive.
These AC trims compensate small
dynamic imbalances that could result
in parasitic bridge outputs.
Conclusion
This concludes part one of this article.
Part two, which will appear in the
November issue of Linear Technology
magazine, details the settling time cir cuitry and presents results. Both parts
represent a distillation of a full-length
LTC application note, AN74, Component and Measurement Advances
Ensure 16-Bit DAC Settling Time.
New 16-Bit Bipolar Output DAC in
Narrow SO-16 Package
by Hassan Malik
Linear Technology introduces its
first bipolar, voltage output 16-bit
digital to analog converter, the
LTC1650. The LTC1650 is available
in a narrow 16-pin SO package, making it the smallest bipolar, 16-bit
voltage output DAC on the market
today. The LTC1650 operates from
± 5V supplies and draws 5mA. It is
equipped with a rail-to-rail, low noise,
deglitched output amplifier that can
be configured to operate in a unipolar
or bipolar mode. The mid-scale glitch
is under 2nV-s and the full-scale
settling time in unipolar mode is 4µ s.
The LTC1650 is 16-bit monotonic
over the industrial temperature range,
5V
3
8
CLK
7
DOUT
5V
MICROWIRE is a trademark of National Semiconductor
Corp.
4.096V
DVDD
11
RSTOUT
REFHI
15
10
AVDD
1.0
0.8
POWER-ON RESET
SUPPLY SENSE
16-BIT DAC REGISTER
5
16-BIT SHIFT REGISTER
DIN
cleared. There are supply brown-out
detectors on all three supplies, AVDD,
DVDD and AV SS. When any of these
supplies drops below 2.5V, the part is
cleared, connecting the output to VRST,
and the RSTOUT pin changes to a
logic low.
The 3-wire serial interface of the
LTC1650 is SPI/QSPI and MICROWIRE™ compatible. All the logic
inputs are TTL/CMOS compatible and
the CLK input is equipped with a
Schmitt trigger that allows direct optocoupler interfacing. There is also a
DOUT pin for daisy-chaining several
DACs. The digital feedthrough is
0.05nV-s.
9
CLR
2
VRST
16-BIT DAC
+
1
VOUT
–
0.6
DNL ERROR (LSB)
CS/LD
with a typical differential nonlinearity of less than ±0.3LSB. Figures 1
and 2 show a typical application for
the part and its DNL curve. The
LTC1650 is equipped with an outputspan-setting resistor tied to the UNI/
BIP pin. When this pin is tied to the
VOUT pin, the output will swing from
REFLO to REFHI; when the pin is tied
to REFHI, the output swings from
–REFHI to REFHI.
The LTC1650 has a user-defined
voltage to which its output resets on
power-up or when the part is cleared.
The voltage on the VRST pin is applied
to the output through a transmission
gate when the part powers up or is
0.4
0.2
0
– 0.2
– 0.4
– 0.6
– 0.8
– 1.0
6
0
16
4
DGND
12,13
REFLO
14
– 5V
Figure 1. LTC1650 block diagram
Linear Technology Magazine • August 1998
AVSS
UNI/BIP
16384
32768
CODE
49152
65535
1650 TA02
1650 TA01
Figure 2. The LTC1650 bipolar output DAC
has ±0.3LSB typical DNL.
33
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