LTC2442IG Linear Technology, LTC2442IG Datasheet - Page 25

• Manufacturer Part Number: LTC2442IG
• Description: IC,A/D CONVERTER,SINGLE,24-BIT,CMOS,SSOP,36PIN
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Optimizing Linearity
While the integrated op-amp has rail-to-rail input range, in
order to achieve parts-per-million linearity performance,
the input range and op-amp supply voltages must be con-
sidered. Input levels within 1.25V of the upper op-amp rail
(V
Figure 15) while operating with V
input voltages (V
2.5V and –2.5V < V
to about 17-bits. Once V
+
) begin to degrade the performance. For example (see
Figure 15. INL vs Op-Amp Supply Voltage
–0.5
–1.5
–10
1.5
0.5
10
–2
–4
–6
–8
–1
–2
8
6
4
2
0
–1.25
2
1
0
–2.5
Figure 17. Linearity Near Ground
V
V
V
V
V
V
V
V
V
REF
CC
INCM
CC
REF
INCM
+
–
IN
+
– 2
= 5V
= –100mV
> 5.25, V
IN
= 5V
= 5V
= 5V
CM + V
= 5V
U
–0.75
– 1.5
= 2.5V
= 0.625V
DIFF < 2.5V), the linearity is degraded
DIFFERENTIAL INPUT (V)
DIFFERENTIAL V
– 1
–
+
V
= 0, –1, –2
+
is increased to 5.25V or greater
–0.25
– 0.5
U
= 5, V
IN
V
V
DIFF) up to 3.75V (V
+
+
0
= 5, V
= 5.1V, V
–
= –2
0.25
0.5 1
–
IN
+
= 0
(V)
W
–
= 5.1V and absolute
= 0
1.5
0.75
2
2442 F15
2442 F17
1.25
2.5
U
IN
CM =
the linearity improves to 19-Bits (2ppm). If the reference is
reduced to 4.096V and the input common mode is V
(2.048V) the linearity performance improves to better
than 1ppm with V
Figure 16. Input signals near ground require about 100mV
headroom on the op-amp power supply in order to achieve
1ppm INL, see Figure 17. Optimal linearity is achieved
by driving the input differentially. As seen in Figure 18, a
single ended input (the negative input is tied to ground)
yields 18-bits (±4ppm) linearity performance. In this case
V
–
is 100mV below ground.
–1
–2
–3
–4
–1
–2
–3
–4
–5
–2.048
4
3
2
1
0
5
4
3
2
1
0
Figure 18. Single-Ended Linearity
0
V
V
V
Figure 16. Linearity vs V
REF
CC
INCM
–1.536
SINGLE ENDED SEL
= 5V
= 4.096V
V
V
V
V
V
V
= 2.048V
CC
REF
IN
IN
+
–
0.5
–1.024
= 5V
= –100mV
–
+
= V
= 5V
= 0V
= 5V
tied to V
DIFFERENTIAL V
IN
–0.512
+
1
0
+
CC
V
, SEL
+
1.5
0.512
= 5V, V
IN
and V
–
(V)
= 0V FIXED
1.024
–
IN
= 0V
2
1.536
–
tied to ground, see
2442 F16
2442 F18
LTC2442
2.048
2.5
25
REF
2442f
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