LTC2641/LTC2642
15
26412fd
For more information www.linear.com/LTC2641
APPLICATIONS INFORMATION
Unbuffered Operation and V
OUT
Loading
The DAC output is available directly at the V
OUT
pin, which
swings from GND to V
REF
. Unbuffered operation provides
the lowest possible offset, full-scale and linearity errors,
the fastest settling time and minimum power consumption.
However, unbuffered operation requires that appropriate
loading be maintained on the V
OUT
pin. The LTC2641/
LTC2642 V
OUT
can be modeled as an ideal voltage source
in series with a source resistance of R
OUT
, typically 6.2k
(Figure 4). The DAC’s linear output impedance allows it to
drive medium loads (R
L
> 60k) without degrading INL or
DNL; only the gain error is increased. The gain error (GE)
caused by a load resistance, R
L
, (relative to full scale) is:
GE =
1+
R
OUT
R
L
In 16-bit LSBs:
GE =
1+
R
OUT
R
L
LSB
R
OUT
has a low tempco (typically < ±50ppm/°C), and is
independent of DAC code. The variation of R
OUT
, part-to-
part, is typically less than ±20%.
Note on LSB units:
For the following error descriptions, “LSB” means 16-bit
LSB and 65,536 is rounded to 66k.
To convert to 14-bit LSBs (LTC2641-14/LTC2642-14)
divide by 4.
To convert to 12-bit LSBs (LTC2641-12/LTC2642-12)
divide by 16.
A constant current, I
L
, loading V
OUT
will produce an offset of:
V
OFFSET
= –I
L
• R
OUT
For V
REF
= 2.5V, a 16-bit LSB equals 2.5V/65,536, or
38µV. Since R
OUT
is 6.2k, an I
L
of 6nA produces an offset
of 1LSB. Therefore, to avoid degrading DAC performance,
it is critical to protect the V
OUT
pin from any sources of
leakage current.
Unbuffered V
OUT
Settling Time
The settling time at the V
OUT
pin can be closely approxi-
mated by a single-pole response where:
t = R
OUT
• (C
OUT
+ C
L
)
(Figure 4). Settling to 1/2LSB at 16-bits requires about
12 time constants (ln(2 • 65,536)). The typical settling
time of 1µs corresponds to a time constant of 83ns, and
a total (C
OUT
+ C
L
) of about 83ns/6.2k = 13pF . The internal
capacitance, C
OUT
is typically 10pF, so an external C
L
of
3pF corresponds to 1µs settling to 1/2LSB.
I
L
V
OUT
0V TO V
REF
R
OUT
V
OUT
C
OUT
LTC2641
LTC2642
REF
REF
GND
CODE
2
N
V
REF
( )
C
L
26412 F04
R
L
+
–
Figure 4. V
OUT
Pin Equivalent Circuit
Op Amp Selection
The optimal choice for an external buffer op amp depends
on whether the DAC is used in the unipolar or bipolar
mode of operation, and also depends on the accuracy,
speed, power dissipation and board area requirements of
the application. The LTC2641/LTC2642’s combination of
tiny package size, rail-to-rail single supply operation, low
power dissipation, fast settling and nearly ideal accuracy
specifications makes it impractical for one op amp type
to fit every application.
In bipolar mode (LTC2642 only), the amplifier operates
with the internal resistors to provide bipolar offset and
scaling. In this case, a precision amplifier operating from
dual power supplies, such as the the LT1678 provides the
±V
REF
output range (Figure 3).
In unipolar mode, the output amplifier operates as a unity
gain voltage follower. For unipolar, single supply applica
-
tions a precision, rail-to-rail input, single supply op amp
