Rainbow Electronics MAX1303 - Manual (Page 25)

• External-acquisition-mode control byte
• Internal-clock-mode control byte
• Reset byte
• Partial power-down-mode control byte
This prevents the MAX1302/MAX1303 from inadvertent-
ly exiting full power-down mode because of a CS glitch
in a noisy digital environment.
Power-On Reset
The MAX1302/MAX1303 power up in normal operation
configured for external clock mode with all circuitry
active (Tables 7 and 8). Each analog input channel
(CH0–CH7) is set for single-ended conversions with a
±V
REF bipolar input range (Table 6).
Allow the power supplies to stabilize after power-up. Do
not initiate any conversions until the power supplies
have stabilized. Additionally, allow 10ms for the internal
reference to stabilize when C
REF = 1.0µF and C RECAP
= 0.1µF. Larger reference capacitors require longer
stabilization times.
Internal or External Reference
The MAX1302/MAX1303 operate with either an internal or
external reference. The reference voltage impacts the
ADC’s FSR (Figures 12, 13, and 14). An external refer-
ence is recommended if more accuracy is required than
the internal reference provides, and/or multiple converters
require the same reference voltage.
Internal Reference
The MAX1302/MAX1303 contain an internal 4.096V
bandgap reference. This bandgap reference is connect-
ed to REFCAP through a nominal 5k
Ω resistor (Figure 17).
The voltage at REFCAP is buffered creating 4.096V at
REF. When using the internal reference, bypass
REFCAP with a 0.1µF or greater capacitor to AGND1 and
bypass REF with a 1.0µF or greater capacitor to AGND1.
External Reference
For external reference operation, disable the internal
reference and reference buffer by connecting REFCAP
to AV
DD1
. With AV
DD1
connected to REFCAP, REF
becomes a high-impedance input and accepts an
external reference voltage. The MAX1302/MAX1303
external reference current varies depending on the
applied reference voltage and the operating mode (see
the External Reference Input Current vs. External
Reference Input Voltage in the Typical Operating
Characteristics).
Applications Information
Noise Reduction
Additional samples can be taken and averaged (over-
sampling) to remove the effect of transition noise on
conversion results. The square root of the number of
samples determines the improvement in performance.
For example, with 2/3 LSB
RMS (4 LSB P-P ) transition
noise, 16 (4 2 = 16) samples must be taken to reduce
the noise to 1 LSB P-P .
Interface with 4–20mA Signals
Figure 19 illustrates a simple interface between the
MAX1302/MAX1303 and a 4–20mA signal. 4–20mA sig-
naling can be used as a binary switch (4mA represents
a logic-low signal, 20mA represents a logic-high sig-
nal), or for precision communication where currents
between 4mA and 20mA represent intermediate analog
data. For binary switch applications, connect the
4–20mA signal to the MAX1302/MAX1303 with a resis-
tor to ground. For example, a 200
Ω resistor converts
the 4–20mA signal to a 0.8V to 4V signal. Adjust the
resistor value so the parallel combination of the resistor
and the MAX1302/MAX1303 source impedance is
200
Ω. In this application, select the single-ended 0 to
V REF range (R[2:0] = 011, Table 6). For applications
that require precision measurements of continuous
analog currents between 4mA and 20mA, use a buffer
to prevent the MAX1302/MAX1303 input from diverting
current from the 4–20mA signal.
MAX1302/MAX1303
8-/4-Channel, ±V
REF
Multirange Inputs,
Serial 16-Bit ADCs
______________________________________________________________________________________ 25
REF
REFCAP
AGND1
4.096V
BANDGAP
REFERENCE
5k Ω
1x
SAR
ADC REF
4.096V
1.0 µF
0.1 µF
V RCTH
MAX1302
MAX1303
Figure 17. Internal Reference Operation