MX7705 datasheet(30/33 Pages) MAXIM

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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 VDD 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

______________________________________________________________________________________

MX7705

VDD

CLKIN

REF+

REF-

AIN1+

AIN1-

CLKOUT

SCLK

DIN

DOUT

DRDY

RESET

GND

VDD

RREF

ACTIVE

GAUGE

DUMMY

GAUGE

0.1

μF

0.1

μF

0.1

μF

VDD = 3V/5V*

*USE A 1.225V REFERENCE FOR VDD = 3V OR A 2.5V REFERENCE FOR VDD = 5V.

VDD

REF+

REF-

AIN1+

AIN1-

GND

CLKIN

CLKOUT

DIN

SCLK

DOUT

THERMOCOUPLE

JUNCTION

1.225V/2.5V

REFERENCE*

μF

0.1

μF

0.1

μF

RESET

DRDY

MX7705

Figure 12. Strain-Gauge Measurement

Figure 13. Temperature Measurement

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