AD7865 查看數據表（PDF） - Analog Devices

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AD7865

Four-Channel, Simultaneous Sampling, Fast, 14-Bit ADC

Analog Devices 

AD7865

CIRCUIT DESCRIPTION

Analog Input Section

The AD7865 is offered as three part types, the AD7865-1 where

each input can be configured for ± 10 V or a ± 5 V input voltage

range, the AD7865-3 which handles input voltage range ± 2.5 V

and the AD7865-2 which has an input voltage range of 0 V to

2.5 V or 0 V to 5 V. The amount of current flowing into the

analog input will depend on the analog input range and the analog

input voltage. The maximum current flows when negative full

scale is applied.

AD7865-1

Figure 2 shows the analog input section of the AD7865-1. Each

input can be configured for ± 5 V or ± 10 V operation on the

AD7865-1. For ± 5 V operation, the VINxA and VINxB inputs are

tied together and the input voltage is applied to both. For ± 10 V

operation, the VINxB input is tied to AGND and the input volt-

age is applied to the VINxA input. The VINxA and VINxB inputs are

symmetrical and fully interchangeable. Thus for ease of PCB

layout on the ± 10 V range, the input voltage may be applied to

the VINxB input while the VINxA input is tied to AGND.

6k⍀

VREF

R2

VINxA

R3

VINxB

2.5V

REFERENCE

AD7865-1

TO ADC

REFERENCE

CIRCUITRY

R1

TRACK/

HOLD

TO INTERNAL

COMPARATOR

R4

GND

Figure 2. AD7865-1 Analog Input Structure

For the AD7865-1, R1 = 4 kΩ, R2 = 16 kΩ, R3 = 16 kΩ and R4

= 8 kΩ. The resistor input stage is followed by the high input

impedance stage of the track/hold amplifier.

The designed code transitions take place midway between suc-

cessive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs

etc.) LSB size is given by the formula, 1 LSB = FSR/16384. For

the ±5 V range, 1 LSB = 10 V/16384 = 610.4 µV. For the ± 10 V

range, 1 LSB = 20 V/16384 = 1.22 mV. Output coding is twos

complement binary with 1 LSB = FSR/16384. The ideal input/

output transfer function for the AD7865-1 is shown in Table I.

Table I. Ideal Input/Output Code Table for the AD7865-1

Analog Input1

+FSR/2 – 3/2 LSB2

+FSR/2 – 5/2 LSB

+FSR/2 – 7/2 LSB

AGND + 3/2 LSB

AGND + 1/2 LSB

AGND – 1/2 LSB

AGND – 3/2 LSB

–FSR/2 + 5/2 LSB

–FSR/2 + 3/2 LSB

–FSR/2 + 1/2 LSB

Digital Output Code Transition

011 . . . 110 to 011 . . . 111

011 . . . 101 to 011 . . . 110

011 . . . 100 to 011 . . . 101

000 . . . 001 to 000 . . . 010

000 . . . 000 to 000 . . . 001

111 . . . 111 to 000 . . . 000

111 . . . 110 to 111 . . . 111

100 . . . 010 to 100 . . . 011

100 . . . 001 to 100 . . . 010

100 . . . 000 to 100 . . . 001

NOTES

1FSR is full-scale range and is 20 V for the ± 10 V range and 10 V for the ± 5 V

range, with VREF = 2.5 V.

21 LSB = FSR/16384 = 1.22 mV (± 10 V—AD7865-1) and 610.4 mV (± 5 V—

AD7865-1) with VREF = 2.5 V.

AD7865-2

Figure 3 shows the analog input section of the AD7865-2. Each

input can be configured for 0 V to 5 V operation or 0 V to 2.5 V

operation. For the 0 V to 5 V operation, the VINxB input is tied

to AGND and the input voltage is applied to VINxA input. For

0 V to 2.5 V operation, the VINxA and VINxB inputs are tied together

and the input voltage is applied to both. The VINxA and VINxB

inputs are symmetrical and fully interchangeable. Thus for ease

of PCB layout on the 0 V to 5 V range the input voltage may be

applied to the VINxB input while the VINxA input is tied to AGND.

For the AD7865-2, R1 = 4 kΩ and R2 = 4 kΩ. Once again,

the designed code transitions occur on successive integer LSB

values. Output coding is straight (natural) binary with 1 LSB

= FSR/16384 = 2.5 V/16384 = 0.153 mV, and 5 V/16384 =

0.305 mV, for 0 V to 2.5 V and 0 V to 5 V options respectively.

Table II shows the ideal input and output transfer function

for the AD7865-2.

6k⍀

VREF

R1

VINxA

R2

VINxB

2.5V

REFERENCE

AD7865-2

TO ADC

REFERENCE

CIRCUITRY

TRACK/

HOLD

TO INTERNAL

COMPARATOR

Figure 3. AD7865-2 Analog Input Structure

Table II. Ideal Input/Output Code Table for the AD7865-2

Analog Input1

Digital Output Code Transition

+FSR/2 – 3/2 LSB2

+FSR/2 – 5/2 LSB

+FSR/2 – 7/2 LSB

111 . . . 110 to 111 . . . 111

111 . . . 101 to 111 . . . 110

111 . . . 100 to 111 . . . 101

AGND + 5/2 LSB

AGND + 3/2 LSB

AGND – 1/2 LSB

000 . . . 010 to 000 . . . 011

000 . . . 001 to 000 . . . 010

000 . . . 000 to 000 . . . 001

NOTES

1FSR is full-scale range and is 0 V to 2.5 V and 0 V to 5 V for AD7865-2 with

VREF = 2.5 V.

21 LSB = FSR/16384 and is 0.153 mV (0 V to 2.5 V) and 0.305 mV (0 V to 5 V)

for AD7865-2) with VREF = 2.5 V.

–10–

REV. B

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