MAX125_08 Datasheet, PDF (11/15 Pages) Maxim Integrated Products – 2x4-Channel, Simultaneous-Sampling 14-Bit DAS

2x4-Channel, Simultaneous-Sampling
14-Bit DAS
OUTPUT CODE
TO DAC
MAX125
MAX126
REFOUT 7 (2.5V)
4.7µF
A_V = 1
REFIN 6 (2.5V)
OUT
MAX6325
10k
2.5V
Figure 7. External Reference
1ppm/°C (max) temperature drift. Connect an external
reference at REFIN as shown in Figure 7. The minimum
impedance is 7kΩ for DC currents in both normal oper-
ation and shutdown. Bypass REFOUT with a 4.7µF low-
ESR capacitor.
Power-On Reset
When power is first applied, the internal power-on-reset
circuitry activates the MAX125/MAX126 with INT =
high, ready to convert. The default conversion mode is
Input Mux A/Single-Channel Conversion. See the
Programming Modes section if other configurations are
desired.
After the power supplies have been stabilized, the reset
time is 5µs; no conversions should be performed
during this phase. At power-up, data in memory is
undefined.
Software Power-Down
Software power-down is activated by setting bit A3 of
the control word high (Table 1). It is asserted after the
WR or CS rising edge, at which point the ADC immedi-
ately powers down to a low quiescent-current state.
AV_DD drops to less than 1.5mA, and AV_SS is reduced
to less than 1mA. The ADC blocks and reference buffer
are turned off, but the digital interface and the refer-
ence remain active for fast power-up recovery. Wake
up the MAX125/MAX126 by writing a control word
(A0–A3, Table 1). The bidirectional interface interprets
a logic zero at A3 as the start signal and powers up in
the mode selected by A0, A1, and A2. The reference
011 . . . 111
011 . . . 110
000 . . . 010
000 . . . 001
000 . . . 000
111 . . . 111
111 . . . 110
111 . . . 101
1LSB = ^4VREFOUT
16384
100 . . . 001
100 . . . 000
- FS
FS = 2 x V_REFOUT (MAX125)
FS = V_REFOUT (MAX126)
ZERO
INPUT VOLTAGE (LSB)
Figure 8. Bipolar Transfer Function
+FS - 1LSB
buffer’s settling time and the bypass capacitor’s value
dominate the power-up delay. With the recommended
4.7µF at REFOUT, the power-up delay is typically 5µs.
Transfer Function
The MAX125/MAX126 have bipolar input ranges. Fig-
ure 8 shows the bipolar/output transfer function. Code
transitions occur at successive-integer least significant
bit (LSB) values. Output coding is twos-complement
binary with 1LSB = 610µV for the MAX125 and
1LSB = 305µV for the MAX126.
Output Demultiplexer
An output demultiplexer circuit is useful for isolating
data from one channel in a four-channel conversion
sequence. Figure 9’s circuit uses the external 16MHz
clock and the INT signal to generate four RD pulses
and a latch clock to save data from the desired chan-
nel. CS must be low during the four RD pulses. The
channel is selected with the binary coding of two
switches. A 16-bit 16373 latch simplifies layout.
Motor-Control Applications
Vector motor control requires monitoring of the individ-
ual phase currents. In their most basic application, the
MAX125/MAX126 simultaneously sample two currents
(CH1A and CH2A, Figure 10) and preserve the neces-
sary relative phase information. Only two of the three
phase currents have to be digitized, because the third
component can be mathematically derived with a coor-
dinate transformation.
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