Rainbow-electronics MAX1326 Manual de usuario descargar pdf (Pagina 20)

MAX1316–MAX1318/MAX1320–MAX1322/MAX1324–MAX1326

8-/4-/2-Channel, 14-Bit, Simultaneous-Sampling ADCs

with ±10V, ±5V, and 0 to +5V Analog Input Ranges

20 ______________________________________________________________________________________

No other digital system ground should be connected to

this single-point analog ground. The ground return to

the power supply for this ground should be low imped-

ance and as short as possible for noise-free operation.

High-frequency noise in the V

power supply may

affect the high-speed comparator in the ADC. Bypass

these supplies to the single-point analog ground with

0.1µF and 2.2µF bypass capacitors close to the device.

If the +5V power supply is very noisy, a ferrite bead can

be connected as a lowpass filter, as shown in Figure 8.

Transfer Functions

Bipolar ±10V Devices

Table 5 and Figure 9 show the two’s complement trans-

fer function for the MAX1324/MAX1325/MAX1326 with a

±10V input range. The full-scale input range (FSR) is

eight times the voltage at REF. The internal +2.500V ref-

erence gives a +20V FSR, while an external +2V to +3V

reference allows an FSR of +16V to +24V, respectively.

Calculate the LSB size using the following equation:

This equals 1.2207mV with a +2.5V internal reference.

The input range is centered about V

MSV

. Normally,

MSV = AGND, and the input is symmetrical about zero.

For a custom midscale voltage, drive MSV with an

external voltage source. Noise present on MSV directly

couples into the ADC result. Use a precision, low-drift

voltage reference with adequate bypassing to prevent

MSV from degrading ADC performance. For maximum

FSR, be careful not to violate the absolute maximum

voltage ratings of the analog inputs when choosing

MSV

Determine the input voltage as a function of V

REF

MSV

, and the output code in decimal using the follow-

ing equation:

Bipolar ±5V Devices

Table 6 and Figure 10 show the two’s complement

transfer function for the MAX1320/MAX1321/MAX1322

with a ±5V input range. The FSR is four times the volt-

age at REF. The internal +2.500V reference gives a

+10V FSR, while an external +2V to +3V reference

allows an FSR of +8V to +12V, respectively. Calculate

the LSB size using the following equation:

This equals 0.6104mV when using the internal reference.

LSB

REF

×4

V LSB CODE V

CH MSV_

=× +

LSB

REF

×8

Figure 9. ±10V Bipolar Transfer Function

8 x V

REF

8 x V

REF

8 x V

REF

1 LSB =

TWO'S COMPLEMENT BINARY OUTPUT CODE

-8192 -8190 +8191+8189

0x2000

0x2001

0x2002

0x2003

0x1FFF

0x1FFE

0x1FFD

0x1FFC

0x3FFF

0x0000

0x0001

-1 0 +1

(MSV)

INPUT VOLTAGE (V

CH_

- V

MSV

IN LSBs)

Table 5. ±10V Bipolar Code Table

TWO’S COMPLEMENT

BINARY OUTPUT CODE

DECIMAL

EQUIVALENT

OUTPUT

(CODE

)

INPUT

VOLTAGE (V)

REF

= 2.5V,

MSV

= 0V)

01 1111 1111 1111

0x1FFF

8191

9.9994

±0.5 LSB

01 1111 1111 1110

0x1FFE

8190

9.9982

±0.5 LSB

00 0000 0000 0001

0x0001

0.0018

±0.5 LSB

00 0000 0000 0000

0x0000

0.0006

±0.5 LSB

11 1111 1111 1111

0x3FFF

-1

-0.0006

±0.5 LSB

10 0000 0000 0001

0x2001

-8191

-9.9982

±0.5 LSB

10 0000 0000 0000

0x2000

-8192

-9.9994

±0.5 LSB

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Rainbow-electronics MAX1326 Manual de usuario Pagina 20

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