LT1679 Linear Technology, LT1679 Datasheet - Page 14

• Manufacturer Part Number: LT1679
• Description: Dual/Quad Low Noise Rail-to-Rail Precision Op Amps
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LTC2641/LTC2642
Unbuffered Operation and V
The DAC output is available directly at the V
swings from GND to V
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
LTC2642 V
in series with a source resistance of R
(Figure 4). The DAC’s linear output impedance allows
it to drive medium loads (R
INL or DNL; only the gain error is increased. The gain
error (GE) caused by a load resistance, R
full scale) is:
In 16-bit LSBs:
R
independent of DAC code. The variation of R
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
of:
For V
Since R
1LSB. Therefore, to avoid degrading DAC performance,
14
OUT
V
GE
GE
OFFSET
REF
has a low tempco (typically < ±50ppm/°C), and is
=
=
OUT
= 2.5V, a 16-bit LSB equals 2.5V/65,536, or 38μV.
1
1
+
+
–65536
OUT
= –I
⎛
⎝ ⎜
⎛
⎝ ⎜
is 6.2k, an I
R
R
–1
R
R
can be modeled as an ideal voltage source
OUT
OUT
L
L
L
• R
⎞
⎠ ⎟
⎞
⎠ ⎟
OUT
L
, loading V
[ ]
LSB
REF
L
. Unbuffered operation provides
of 6nA produces an offset of
L
OUT
> 60k) without degrading
OUT
OUT
Loading
will produce an offset
pin. The LTC2641/
OUT
, typically 6.2k
OUT
L
, (relative to
OUT
pin, which
, part-to-
it is critical to protect the V
leakage current.
Unbuffered V
The settling time at the V
mated by a single-pole response where:
(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
capacitance, C
3pF corresponds to 1μs settling to 1/2LSB.
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
specifi cations makes it impractical for one op amp type
to fi t every application.
In bipolar mode (LTC2642 only), the amplifi er operates
with the internal resistors to provide bipolar offset and
scaling. In this case, a precision amplifi er operating from
dual power supplies, such as the the LT1678 provides the
±V
In unipolar mode, the output amplifi er operates as a unity
gain voltage follower. For unipolar, single supply applica-
tions a precision, rail-to-rail input, single supply op amp
τ = R
REF
V
LTC2641
LTC2642
REF
output range (Figure 3).
OUT
( )
OUT
CODE
2
N
• (C
Figure 4. V
+ C
–
OUT
+
OUT
GND
V
OUT
L
REF
R
REF
) of about 83ns/6.2k = 13pF. The internal
OUT
Settling Time
is typically 10pF, so an external C
+ C
OUT
L
C
V
)
OUT
OUT
OUT
Pin Equivalent Circuit
OUT
pin can be closely approxi-
R
pin from any sources of
L
www.DataSheet4U.com
C
L
26412 F04
I
L
V
0V TO V
OUT
REF
26412f
L
of