Rainbow Electronics MX7705 - Manual (Page 30)

MX7705
Applications Information
Applications Examples
Strain-Gauge Measurement
Connect the differential inputs of the MX7705 to the
bridge network of the strain gauge. In Figure 12, the ana-
log positive supply voltage powers the bridge network
and the MX7705, along with the reference voltage in a
ratiometric configuration. The on-chip PGA allows the
MX7705 to handle an analog input voltage range as low
as 20mV to full scale.
Temperature Measurement
Use the MX7705 for temperature measurements from a
thermocouple (Figure 13). Operate the MX7705 in
buffered mode to allow large decoupling capacitors at
the analog inputs. The decoupling capacitors eliminate
any noise pickup from the thermocouple leads. AIN1- is
biased up at the reference voltage to accommodate the
reduced common-mode input range in buffered mode.
Optical Isolation
For applications that require an optically isolated inter-
face, see Figure 14. With 6N136-type optocouplers,
maximum clock speed is 4MHz. Maximum clock speed
is limited by the degree of mismatch between the indi-
vidual optocouplers. Faster optocouplers allow faster
signaling at a higher cost.
Layout, Grounding, Bypassing
Use PC boards with separate analog and digital
ground planes. Connect the two ground planes togeth-
er at the MX7705 GND. Isolate the digital supply from
the analog with a low-value resistor (10
Ω) or ferrite
bead when the analog and digital supplies come from
the same source.
Ensure that digital return currents do not pass through
the analog ground and that return-current paths are low
impedance. A 5mA current flowing through a PC board
ground trace impedance of only 0.05
Ω creates an error
voltage of about 250µV.
Layout the PC board to ensure digital and analog sig-
nal lines are kept separate. Do not run digital lines
(especially the SCLK and DOUT) parallel to any analog
lines. If they must cross another, do so at right angles.
Bypass V DD to the analog ground plane with a 0.1µF
capacitor in parallel with a 1µF to 10µF low-ESR capac-
itor. Keep capacitor leads short for best supply-noise
rejection. Bypass REF+ and REF- with a 0.1µF capaci-
tor to GND. Place all bypass capacitors as close to the
device as possible to achieve the best decoupling.
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
30 ______________________________________________________________________________________
MX7705
V DD
CLKIN REF+
REF-
AIN1+
AIN1-
CLKOUT
CS
SCLK
DIN
DOUT
DRDY
RESET
GND
V DD
R REF
ACTIVE
GAUGE
DUMMY
GAUGE
0.1 µF
10 µF
0.1 µF
0.1 µF
R
R
V DD = 3V/5V*
*USE A 1.225V REFERENCE FOR V DD = 3V OR A 2.5V REFERENCE FOR V DD = 5V.
V DD
REF+
REF-
AIN1+
AIN1-
GND
CLKIN
CLKOUT
DIN
SCLK
DOUT
THERMOCOUPLE
JUNCTION
1.225V/2.5V
REFERENCE*
10 µF
10 µF
0.1 µF
0.1 µF
RESET
DRDY
CS MX7705
Figure 12. Strain-Gauge Measurement
Figure 13. Temperature Measurement