Datasheet

LTC2442
25
2442fa
For more information www.linear.com/LTC2442
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
+
) begin to degrade the performance. For example (see
Figure 15) while operating with V
+
= 5.1V and absolute
input voltages (V
IN
CM + V
IN
DIFF) up to 3.75V (V
IN
CM =
2.5V and –2.5V < V
IN
DIFF < 2.5V), the linearity is degraded
to about 17-bits. Once V
+
is increased to 5.25V or greater
the linearity improves to 19-Bits (2ppm). If the reference is
reduced to 4.096V and the input common mode is V
REF
/2
(2.048V) the linearity performance improves to better
than 1ppm with V
+
tied to V
CC
and V
tied to ground, see
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.
DIFFERENTIAL V
IN
(V)
–2.5
INL (ppm)
2
6
10
1.5
2442 F15
–2
–6
0
4
8
–4
–8
–10
1.52
0.51
0.5 1 2
0
2.5
V
+
= 5.1V, V
= 0
V
+
= 5, V
= 0
V
+
= 5, V
= –2
V
REF
= 5V
V
CC
= 5V
V
INCM
= 2.5V
V
+
> 5.25, V
= 0, –1, –2
DIFFERENTIAL V
IN
(V)
–2.048
INL (ppm)
0
2
2.048
2442 F16
–2
–4
–1.024
0
1.024
–1.536
–0.512
0.512
1.536
4
–1
1
–3
3
V
REF
= 4.096V
V
CC
= 5V
V
INCM
= 2.048V
V
+
= 5V, V
= 0V
Figure 15. INL vs Op-Amp Supply Voltage
Figure 16. Linearity vs V
IN
DIFFERENTIAL INPUT (V)
–1.25
INL (ppm)
–0.5
0
0.5
0.25
1.25
2442 F17
–1
–1.5
–2
–0.75 –0.25 0.75
1
1.5
2
V
CC
= 5V
V
REF
= 5V
V
INCM
= 0.625V
V
+
= 5V
V
= –100mV
SINGLE ENDED SEL
+
, SEL
= 0V FIXED
0
INL (ppm)
1
3
5
2
2442 F18
–1
–3
0
2
4
–2
–4
–5
0.5
1
1.5
2.5
V
CC
= 5V
V
REF
= 5V
V
IN
= V
IN
+
V
IN
= 0V
V
+
= 5V
V
= –100mV
Figure 17. Linearity Near Ground Figure 18. Single-Ended Linearity
applications inForMation