Rainbow Electronics MAX194 - Manual (Page 12)

MAX194
be improved by trimming, but the drift is too great to
provide good stability over temperature. The MAX427
buffer provides the necessary drive current to stabilize
the REF input quickly after capacitance changes.
The reference inaccuracies contribute additional full-
scale error. A reference with less than
1 ⁄
2
14 total error
(61 parts per million) over the operating temperature
range is required to maintain full 14-bit accuracy. The
MAX6241 achieves a drift specification of 1ppm/°C
(typ) and easily drives the REF input directly. This
allows large temperature changes with less than 1LSB
error. While the MAX6241’s initial-accuracy specifica-
tion (0.02%) results in a maximum error of about
±4LSB, the reference voltage can be trimmed or the
offset can be corrected in software if absolute DC
accuracy is essential. Figure 11’s circuit provides out-
standing temperature stability and also provides excel-
lent DC accuracy if the initial error is corrected.
14-Bit, 85ksps ADC with 10µA Shutdown
12 ______________________________________________________________________________________
14
16
15
12
MAX194
MAX427
AGND VSSA
VDDA
4
4
7
6
8
6
2
2
3
MAX874
GND
V IN
COMP
V OUT
4.096V
+15V
-15V
47 µ F
LOW
ESR
0.1 µ F
0.1 µ F
0.1 µ F
0.1 µ F
1000pF
0.1 µ F
0.1 µ F
1k
10 Ω
2k
REF
1N914
1N914
10 Ω
-5V
+5V
6 12
5
4 14
3
2.2 µ F
V
IN
≥ 8V
1 µ F
10k 2.2 µ F 0.1 µ F
2
MAX6241
IN
OUT REF
TRIM NR
GND AGND
MAX194
Figure 10. Typical Reference Circuit for AC Accuracy
Figure 11. High-Accuracy Reference