which can be achieved using power-down between
conversions.
External Compensation
Figure 19a shows the connection for external compensa-
tion with reference adjustment. In this mode, an external
4.7µF capacitor compensates the reference output
amplifier, allowing for maximum conversion speed and
lowest conversion noise. However, when reactivating the
ADC after power-down, the reference takes typically 2ms
to fully charge the 4.7µF capacitor, so more time is
required before a conversion can start (Figure 19b).
Thus, the average current consumed in power-up/power-
down operations is higher in external compensation
mode than in internal compensation mode.
Gain and Offset Adjustment
Figure 20 depicts the nominal, unipolar input/output (I/O)
transfer function, and Figure 22 shows the bipolar I/O
transfer function. Code transitions occur halfway between
successive integer LSB values. Note that 1LSB = 1.00mV
(4.096V/4096) for unipolar operation and 1LSB = 1.00mV
((4.096V/2 - -4.096V/2)/4096) for bipolar operation.
Figures 19a and 21a show how to adjust the ADC gain
in applications that require full-scale range adjustment.
The connection shown in Figure 21a provides ±0.5%
for ±20LSBs of adjustment range and is recommended
for applications that use an external reference. On the
other hand, Figure 19a is recommended for applica-
tions that use the internal reference, because it uses
fewer external components.
If both offset and full scale need adjustment, the circuit
in Figure 21b is recommended. For single-supply
ADCs, it is virtually impossible to null system negative
offset errors. However, the MAX191 input configuration
is pseudo-differential—only the difference in voltage
between AIN+ and AIN- will be converted into its digital
representation. By applying a small positive voltage to
AIN-, the 0 input voltage at AIN+ can be adjusted to
above or below AIN- voltage, thus nulling positive or
negative system offset errors. R9 and R10 can be
removed for applications that require only positive sys-
tem errors to be nulled. To trim the offset error of the
MAX191, apply 1/2LSB to the analog input and adjust
R6 so the digital output code changes between 000
(hex) and 001 (hex). To adjust full scale, apply FS - 1
1/2LSBs and adjust R2 until the output code changes
MAX191
Low-Power, 12-Bit Sampling ADC
with Internal Reference and Power-Down
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