MAX194 Maxim, MAX194 Datasheet - Page 11

• Manufacturer Part Number: MAX194
• Description: 14-Bit / 85ksps ADC with 10A Shutdown
• Manufacturer: Maxim

Table 1. Low-ESR Capacitor Suppliers
Figure 9. Ratiometric Measurement Without an Accurate Reference
The MAX194 reference voltage range is 0V to VDDA.
When choosing the reference voltage, the MAX194’s
equivalent input noise (40µV
80µV
V
protection diodes that will begin to conduct, so use cau-
tion when using a reference near VDDA (unless V
and VDDA are virtually identical). V
exceed its absolute maximum rating (VDDA + 0.3V).
The MAX194 needs a good reference to achieve its
rated performance. The most important requirement is
that the reference must present a low impedance to the
REF input. This is often achieved by buffering the refer-
ence through an op amp and bypassing the REF input
with a large (1µF to 47µF), low-ESR capacitor in parallel
with a 0.1µF ceramic capacitor. Low-ESR capacitors
are available from the manufacturers listed in Table 1.
The reference must drive the main conversion DAC
capacitors as well as the capacitors in the calibration
__________Applications Information
REF
RMS
exceeds VDDA, errors will occur due to the internal
COMPANY
in bipolar mode) should be considered. Also, if
Nichicon
Sprague
Sanyo
AVX
14-Bit, 85ksps ADC with 10µA Shutdown
______________________________________________________________________________________
BRIDGE
RMS
in unipolar mode,
OS-CON series,
CAPACITOR
MV-GX series
595D series,
592D series
REF
TPS series
PL series
Reference
must never
INSTRUMENTATION
AMPLIFIER
REF
47 F
LOW
ESR
FACTORY FAX [COUNTRY CODE]
DACs, all of which may be switching between GND and
REF at the conversion clock frequency. The total
capacitive load presented can exceed 1000pF and,
unlike the analog input (AIN), REF is sampled continu-
ously throughout the conversion.
The first step in choosing a reference circuit is to
decide what kind of performance is required. This often
suggests compromises made in the interests of cost
and size. It is possible that a system may not require an
accurate reference at all. If a system makes a ratiomet-
ric measurement such as Figure 9’s bridge circuit, any
relatively noise-free voltage that presents a low imped-
ance at the REF input will serve as a reference. The
+5V analog supply suffices if you use a large, low-
impedance bypass capacitor to keep REF stable dur-
ing switching of the capacitor arrays. Do not place a
resistance between the +5V supply and the bypass
capacitor, because it will cause linearity errors due to
the dynamic REF input current, which typically ranges
from 300µA to 400µA.
Figure 10 shows a more typical scheme that provides
good AC accuracy. The MAX874’s initial accuracy can
[1]-603-224-1430
[1]-207-283-1941
[81]-7-2070-1174
[1]-708-843-2798
0.1 F
CERAMIC
REF
AIN
MAX194
VDDA
+5V
USA TELEPHONE
603-224-1961
800-282-4975
619-661-6835
708-843-7500
11