MAX1226 Datasheet, PDF (11/24 Pages) Maxim Integrated Products – 12-Bit 300ksps ADCs with FIFO, Temp Sensor, Internal Reference

12-Bit 300ksps ADCs with FIFO,
Temp Sensor, Internal Reference
AIN0-AIN15
(SINGLE ENDED);
AIN0, AIN2,
AIN4…AIN14
(DIFFERENTIAL)
HOLD
GND
(SINGLE ENDED);
AIN1, AIN3,
AIN5…AIN15
(DIFFERENTIAL)
REF
GND
DAC
CIN+
CIN-
HOLD
V_DD/2
COMPARATOR
+
-
HOLD
Figure 3. Equivalent Input Circuit
appropriate bit of the bipolar or unipolar register.
Unipolar mode sets the differential input range from 0 to
V_REF. A negative differential analog input in unipolar
mode causes the digital output code to be zero.
Selecting bipolar mode sets the differential input range
to ±V_REF / 2. The digital output code is binary in unipo-
lar mode and two’s complement in bipolar mode. (See
the transfer function graphs, Figures 8 and 9.)
In single-ended mode, the MAX1226/MAX1228/
MAX1230 always operate in unipolar mode. The analog
inputs are internally referenced to GND with a full-scale
input range from 0 to V_REF.
True Differential Analog Input T/H
The equivalent circuit of Figure 3 shows the
MAX1226/MAX1228/MAX1230s’ input architecture. In
track mode, a positive input capacitor is connected to
AIN0–AIN15 in single-ended mode (and AIN0, AIN2,
AIN4…AIN14 in differential mode). A negative input
capacitor is connected to GND in single-ended mode
(or AIN1, AIN3, AIN5…AIN15 in differential mode). For
external track-and-hold timing, use clock mode 01.
After the T/H enters hold mode, the difference between
the sampled positive and negative input voltages is
converted. The time required for the T/H to acquire an
input signal is determined by how quickly its input
capacitance is charged. If the input signal’s source
impedance is high, the required acquisition time length-
ens. The acquisition time, t_ACQ, is the maximum time
needed for a signal to be acquired, plus the power-up
time. It is calculated by the following equation:
( ) ^t_AQC ^{= 9 x R}_S ^{+ R}_IN ^{x 24pF + t}_PWR
where R_IN = 1.5kΩ, R_S is the source impedance of the
input signal, and t_PWR = 1µs, the power-up time of the
device. The varying power-up times are detailed in the
explanation of the clock mode conversions.
t_ACQ is never less than 1.4µs, and any source imped-
ance below 300Ω does not significantly affect the
ADC’s AC performance. A high-impedance source can
be accommodated either by lengthening t_ACQ or by
placing a 1µF capacitor between the positive and neg-
ative analog inputs.
Internal FIFO
The MAX1226/MAX1228/MAX1230 contain a FIFO
buffer that can hold up to 16 ADC results plus one tem-
perature result. This allows the ADC to handle multiple
internally clocked conversions and a temperature mea-
surement, without tying up the serial bus.
If the FIFO is filled and further conversions are request-
ed without reading from the FIFO, the oldest ADC
results are overwritten by the new ADC results. Each
result contains 2 bytes, with the MSB preceded by 4
leading zeros. After each falling edge of CS, the oldest
available byte of data is available at DOUT, MSB first.
When the FIFO is empty, DOUT is zero.
The first 2 bytes of data read out after a temperature mea-
surement always contain the temperature result preceded
by 4 leading zeros, MSB first. If another temperature mea-
surement is performed before the first temperature result
is read out, the old measurement is overwritten by the
new result. Temperature results are in degrees Celsius
(two’s complement) at a resolution of 1/8 of degree. See
the Temperature Measurements section for details on
converting the digital code to a temperature.
Internal Clock
The MAX1226/MAX1228/MAX1230 operate from an inter-
nal oscillator, which is accurate within 10% of the 4.4MHz
nominal clock rate. The internal oscillator is active in clock
modes 00, 01, and 10. Read out the data at clock speeds
up to 10MHz. See Figures 4–7 for details on timing speci-
fications and starting a conversion.
Applications Information
Register Descriptions
The MAX1226/MAX1228/MAX1230 communicate
between the internal registers and the external circuitry
through the SPI-/QSPI-compatible serial interface.
Table 1 details the registers and the bit names. Tables
2–7 show the various functions within the conversion
register, setup register, averaging register, reset regis-
ter, unipolar register, and bipolar register.
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