LTC2757 Linear Technology, LTC2757 Datasheet - Page 15

• Manufacturer Part Number: LTC2757
• Description: 18-Bit SoftSpan IOUT DAC
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
Op Amp Selection
Because of the extremely high accuracy of the 18-bit
LTC2757, careful thought should be given to op amp
selection in order to achieve the exceptional performance
of which the part is capable. Fortunately, the sensitivity of
INL and DNL to op amp offset has been greatly reduced
compared to previous generations of multiplying DACs.
Tables 3 and 4 contain equations for evaluating the ef-
fects of op amp parameters on the LTC2757’s accuracy.
These are the changes the op amp can cause to the INL,
DNL, unipolar offset, unipolar gain error, bipolar zero and
bipolar gain error.
Table 4. Easy-to-Use Equations Determine Op Amp Effects on DAC Accuracy in All Output Ranges (Circuit of Page 1).
Subscript 1 Refers to Output Amp, Subscript 2 Refers to Reference Inverting Amp.
(V/mV)
(V/mV)
A
A
(mV)
(mV)
AMP
V
V
(nA)
(nA)
OP
I
VOL1
I
VOL2
OS1
OS2
B1
B2
I
V
B1
OS1
•0.0012•
A1•
•12.1•
INL (LSB)
⎛
⎜
⎝
A
0
0
0
66
VOL1
⎛
⎝ ⎜
⎛
⎝ ⎜
V
V
5V
REF
5V
REF
⎞
⎟
⎠
⎞
⎠ ⎟
⎞
⎠ ⎟
I
B1
V
•0.00032•
OS1
A2•
DNL (LSB)
•3.1•
⎛
⎜
⎝
A
0
0
0
VOL1
⎛
⎝ ⎜
6
V
⎛
⎝ ⎜
5V
REF
V
5V
⎞
⎟
⎠
REF
⎞
⎠ ⎟
⎞
⎠ ⎟
A3•V
I
B1
OFFSET (LSB)
•0.524•
OS1
UNIPOLAR
•52.4•
0
0
0
0
⎛
⎝ ⎜
V
⎛
⎝ ⎜
5V
REF
V
5V
REF
⎞
⎠ ⎟
⎞
⎠ ⎟
Table 3. Coeffi cients for the Equations of Table 4
BIPOLAR ZERO ERROR
A3•V
A4•V
A4•I
OUTPUT RANGE
I
–2.5V to 7.5V
B1
B2
A4•
OS1
•0.524•
OS2
±2.5V
±10V
•0.524•
10V
±5V
5V
•78.6•
•52.4•
⎛
⎜
⎝
(LSB)
A
0
262
VOL2
⎛
⎝ ⎜
V
⎛
⎝ ⎜
⎛
⎝ ⎜
⎛
⎝ ⎜
5V
REF
V
V
V
⎞
⎟
⎠
5V
5V
REF
5V
REF
REF
⎞
⎠ ⎟
⎞
⎠ ⎟
⎞
⎠ ⎟
⎞
⎠ ⎟
1.1
2.2
1.9
A1
V
2
4
1
I
V
I
B1
OS2
B2
OS1
UNIPOLAR GAIN
•0.0072•
A5•
ERROR (LSB)
•1.048•
•104.8•
•52.4•
⎛
⎜
⎝
A
⎛
⎜
⎝
524
VOL2
A2
A
2
3
2
4
1
3
524
VOL1
⎛
⎝ ⎜
⎛
⎝ ⎜
⎛
⎝ ⎜
⎛
⎝ ⎜
V
V
⎞
⎟
⎠
V
5V
V
REF
5V
REF
5V
REF
5V
REF
⎞
⎟
⎠
⎞
⎠ ⎟
0.83
⎞
⎠ ⎟
0.5
1.4
0.7
⎞
⎠ ⎟
A3
⎞
⎠ ⎟
1
1
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LTC2757
V
V
I
I
B1
OS2
B2
OS1
BIPOLAR GAIN
•0.0072•
ERROR (LSB)
A5•
•1.048•
•104.8•
•52.4•
⎛
⎜
⎝
0.5
A4
1
1
1
A
⎛
⎜
⎝
524
VOL2
A
524
VOL1
⎛
⎝ ⎜
⎛
⎝ ⎜
⎛
⎝ ⎜
15
⎛
⎝ ⎜
V
V
⎞
⎟
⎠
V
5V
5V
V
REF
REF
5V
REF
5V
REF
1.5
1.5
2.5
1.5
⎞
⎟
⎠
A5
1
1
2757f
⎞
⎠ ⎟
⎞
⎠ ⎟
⎞
⎠ ⎟
⎞
⎠ ⎟