Rainbow Electronics MX7705 - Manual (Page 17)

Detailed Description
The MX7705 low-power, 2-channel, serial-output ADC
uses a sigma-delta modulator with a digital filter to
achieve 16-bit resolution with no missing codes. The
device includes a PGA, an on-chip input buffer, and a
bidirectional communications port. The MX7705 oper-
ates with a single 2.7V to 5.25V supply.
Fully differential inputs, an internal input buffer, and an
on-chip PGA (gain = 1 to 128) allow low-level signals to
be directly measured, minimizing the requirements for
external signal conditioning. Self-calibration corrects for
gain and offset errors. A programmable digital filter
allows for the selection of the output data rate and first-
notch frequency from 20Hz to 500Hz.
The bidirectional serial SPI-/QSPI-/MICROWIRE-compati-
ble interface consists of four digital control lines (SCLK,
CS, DOUT, and DIN) and provides an easy interface to
microcontrollers (µCs). Connect CS to GND to configure
the MX7705 for 3-wire operation.
Analog Inputs
The MX7705 accepts four analog inputs (AIN1+, AIN1-,
AIN2+, and AIN2-) in buffered or unbuffered mode.
Use Table 8 to select the positive and negative input
pair for a fully differential channel. The input buffer iso-
lates the inputs from the capacitive load presented by
the PGA/modulator, allowing for high source-imped-
ance analog transducers. The value of the BUF bit in
the setup register (see the Setup Register section) deter-
mines whether the input buffer is enabled or disabled.
Internal protection diodes, which clamp the analog
input to V
DD and/or GND, allow the input to swing from
(GND - 0.3V) to (V DD + 0.3V), without damaging the
device. If the analog input exceeds 300mV beyond the
supplies, limit the input current to 10mA.
Input Buffers
When the analog input buffer is disabled, the analog
input drives a typical 7pF (gain = 1) capacitor, C
TOTAL ,
in series with the 7k Ω typical on-resistance of the track
and hold (T/H) switch (Figure 1). C TOTAL is comprised
of the sampling capacitor, C SAMP , and the stray capac-
itance, C
STRAY
. During the conversion, C
SAMP
charges
to (AIN+ - AIN-). The gain determines the value of
C
SAMP (Table 5).
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
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MX7705
SWITCHING
NETWORK
CLOCK
GENERATOR
DIVIDER
DIGITAL
FILTER
2ND-ORDER
SIGMA-DELTA
MODULATOR
PGA
BUFFER
BUFFER
SERIAL INTERFACE,
REGISTERS,
AND
CONTROL
V
DD
GND
SCLK
DIN
DOUT
CLKIN
CLKOUT
AIN1+
AIN1-
AIN2+
AIN2-
REF+
REF-
S1
S2
S1 AND S2 ARE OPEN IN
BUFFERED MODE AND CLOSED
IN UNBUFFERED MODE
CS
DRDY
RESET
Functional Diagram