MAX110AEWE+ Datasheet MAX111BCPE+, MAX111BEAP+, MAX111BEWE+ | P19-P21

MAX110/MAX111

Low-Cost, 2-Channel, ±14-Bit Serial ADCs

______________________________________________________________________________________ 19

A 100ms conversion time cannot be achieved with either

10,240 CCPC or 20,480 CCPC modes because f

OSC

would be below the minimum 250kHz requirement.

When the gain calibration is performed, the conversion

times change approximately 1% to compensate for the

modulator’s gain error. This slightly degrades the line-

frequency rejection, because the corrected conversion

time is no longer an exact multiple of the line frequency.

Typically, the rejection of 50Hz/60Hz from the converter

is 55dB; i.e., if there is 100mV injection at the reference

or the analog input pin, it will cause an uncertainty of

±0.006%. If the system has large 50Hz/60Hz noise, the

use of internal auto gain calibration is not recommend-

ed. Instead, gain calibration should be done off-chip,

using numerical computation methods.

If you wish to use a configuration other than those sug-

gested in Table 6, you can accomplish similar 50Hz

and 60Hz line-frequency rejection off-chip by averag-

ing several conversions.

__________Applications Information

Layout, Grounding, Bypassing

For minimal noise, bypass each supply to GND with a

0.1µF capacitor. A ground plane should also be placed

under the analog circuitry. To minimize the coupling

effects of stray capacitance, keep digital lines as far

from analog components and lines as possible. Figure

10 shows the suggested power-supply and ground-

plane connections.

*R = 10Ω

*OPTIONAL

DIGITAL

CIRCUITRY

POWER

SUPPLIES

+5V DGND

+5V -5V GND

GND

4.7µF

0.1µF

4.7µF

MAX110

Figure 10a. MAX110 Power-Supply Grounding Connections

*R = 10Ω

*OPTIONAL

DIGITAL

CIRCUITRY

POWER

SUPPLIES

AGND +5V DGND

+5V

GND

4.7µF

0.1µF

MAX111

Figure 10b. MAX111 Power-Supply Grounding Connections

CCPC = Clock Cycles per Conversion

Table 6. Suggested XCLK Frequencies to Achieve Maximum Rejection of Both 50Hz/60Hz Line

Frequencies

MAX111 (t

CONVERT

= 200ms)

81,240 CCPC 102,400 CCPC

DIVIDER

RATIO

XCLK

(MHz)

RELATIVE

ACCURACY

(%)

XCLK

(MHz)

RELATIVE

ACCURACY

(%)

1:1 0.4062 0.030 0.512 0.030

2:1 0.8124 0.025 1.024 0.025

4:1 1.6248 0.022 2.048 0.023

MAX110 (t

CONVERT

= 100ms)

81,240 CCPC 102,400 CCPC

DIVIDER

RATIO

XCLK

(MHz)

RELATIVE

ACCURACY

(%)

XCLK

(MHz)

RELATIVE

ACCURACY

(%)

1:1 0.8124 0.025 1.024 0.065

2:1 1.6248 0.018 2.048 0.045

4:1 3.2496 0.016 4.096 0.030

MAX110/MAX111

Low-Cost, 2-Channel, ±14-Bit Serial ADCs

20 ______________________________________________________________________________________

Capacitive Loading Effects of XCLK in

Internal RC-Oscillator Mode

When using the internal RC oscillator, capacitive load-

ing effects on the XCLK pin must be minimized. Stray

capacitance causes the V

power consumption to

increase by an amount p =

⁄

f, where C = stray

capacitance, V is the supply voltage, and f is the fre-

quency of the internal RC oscillator.

External Reference

The reference inputs to the ADC are high impedance,

allowing both an external voltage reference and ratio-

metric applications without loading effects. The fully dif-

ferential analog signal and reference inputs are

advantageous for performing ratiometric conversions

(Figures 11 and 12). For example, when measuring

load cells, the bridge excitation and the ADC reference

input both share the same voltage source. As the exci-

tation changes with temperature or voltage, the output

of the load cell will change. But since the differential

reference voltage also changes, the conversion results

remain constant, all else remaining equal.

Weigh Scale Application

The fully differential analog signal and reference inputs

make the MAX111 easy to interface to transducers with

differential outputs, such as the load cell in Figure 11.

Because the ADC input is differential, the load cell only

requires differential gain, eliminating the need for the

difference amplifier (differential to single-ended con-

verter) of the standard three op-amp instrumentation-

amplifier realization.

The 30mV full-scale bridge output is amplified to 2V

full-scale and applied to the MAX111 channel-one

input. The reference voltage to the ADC is created by a

voltage divider connected to the +5V rail. The same 5V

provides excitation for the bridge; therefore, as the

excitation voltage varies, the reference voltage to the

ADC also varies, providing an ADC output that does

not depend on the supply voltage.

The two 121kΩ resistors connected to the +5V supplies

shift the common-mode voltage from 2.5V (5V/2) to

1.5V to ensure linearity. Match these two resistors to

avoid introducing differential offset, or trim the resistor

mismatch with a potentiometer. In practice, the scale is

“zeroed” or “tared” by storing the average of several

conversions in a memory location while the scale is

+5V

30mV

FULL-SCALE

121k

49.9k

22k

10k

1/2 MAX492

1µF

REF+

REF-

IN1+

IN1-

AGND

DIN

DOUT

SCLK

49.9k

+5V

0.1µF

MAX111

+5V

GND

Figure 11. Weigh Scale Application

unloaded, and subtracting this value from actual weight

measurements. The lowpass filtering action of the

MAX111’s sigma-delta converter helps minimize noise.

The resolution of the weigh scale can be further

increased by averaging several conversions.

Thermocouple Circuit with Software

Compensation

A thermocouple is created by the junction of dissimilar

metals, and generates a voltage proportional to temper-

ature (Seebeck voltage), making it useful for tempera-

ture-measurement instruments. When a thermocouple

probe is connected to a measurement instrument, other

thermoelectric potentials are created between the alloys

of the probe and the copper connectors of the instru-

ment. These potentials introduce a temperature-depen-

dent error that must be subtracted from the temperature

measurement to obtain an accurate result. According to

the law of intermediate metals, the junction of the ther-

mocouple-probe alloys with the copper of the instrument

junction block can be treated as another thermocouple

of the same type. The voltage measured by the instru-

ment can be expressed as:

V = α(T1 - T

REF

)

where α is the Seebeck constant for the type of thermo-

couple, T1 is the temperature being measured, and

REF

is the temperature of the junction block. Although

one method to obtain T

REF

is to force the junction block

to a known temperature (0°C), a more popular

approach is to measure T

REF

directly using a thermistor

or PN junction voltage.

The circuit in Figure 12 shows a k-type thermocouple

going through a 54dB gain stage to channel 1 of the

MAX110. A MAX874 voltage reference provides both

the 3V reference voltage and reference junction tem-

perature information to the MAX110. Armed with the

temperature information provided by the MAX874, the

thermocouple voltage created at the junction block can

be subtracted out in software. The TEMP output of the

MAX874 is nominally 690mV at room temperature, and

increases with temperature at about 2.3mV/°C. Place

the MAX874 as close as possible to the terminal block,

and ensure good thermal contact between them. This

circuit employs a common k-type thermocouple and,

with the component values shown, can indicate tem-

peratures in the range of -150°C to +125°C.

MAX110/MAX111

Low-Cost, 2-Channel, ±14-Bit Serial ADCs

______________________________________________________________________________________ 21

243k

10k

1µF

IN1+

IN1-

REF-

REF+

-5V

DIN

DOUT

SCLK

243k

10k 10k

K-TYPE

+5V

IN2-

IN2+

MAX110

1/4 MAX479

TEMP

OUT

MAX874

+5V

Figure 12. Thermocouple Circuit with Software Compensation

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MAX110AEWE+

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Analog to Digital Converters - ADC 2Ch +/-14Bit Serial

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