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

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19

A 100ms conversion time cannot be achieved with either

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 = 10

Ω

*OPTIONAL

DIGITAL

CIRCUITRY

POWER

SUPPLIES

+5V

DGND

+5V

-5V

GND

GND

4.7

µF

0.1

µF

0.1

µF

4.7

µF

MAX110

Figure 10a. MAX110 Power-Supply Grounding Connections

*R = 10

Ω

*OPTIONAL

DIGITAL

CIRCUITRY

POWER

SUPPLIES

AGND

+5V

DGND

+5V

GND

GND

4.7

µF

0.1

µF

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

81,240 CCPC

102,400 CCPC

DIVIDER

RATIO

(MHz)

RELATIVE

ACCURACY

(%)

(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

81,240 CCPC

102,400 CCPC

DIVIDER

RATIO

(MHz)

RELATIVE

ACCURACY

(%)

(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