AD7891_15 datasheet(12/20 Pages) AD

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AD7891

–12–

REV. D

CIRCUIT DESCRIPTION

Reference

The AD7891 contains a single reference pin labeled REF OUT/

REF IN that either provides access to the part’s own 2.5 V

internal reference or to which an external 2.5 V reference can be

connected to provide the reference source for the part. The part

is specified with a 2.5 V reference voltage. Errors in the reference

source result in gain errors in the transfer function of the AD7891

and add to the specified full-scale errors on the part. They also

result in an offset error injected into the attenuator stage.

The AD7891 contains an on-chip 2.5 V reference. To use this

reference as a reference source for the AD7891, simply connect

a 0.1

mF disc ceramic capacitor from the REF OUT/REF IN pin

to REFGND. REFGND should be connected to AGND or the

analog ground plane. The voltage that appears at the REF OUT/

REF IN pin is internally buffered before being applied to the

ADC. If this reference is required for use external to the AD7891,

it should be buffered since the part has a FET switch in series

with the reference, resulting in a source impedance for this

output of 2 k

W nominal. The tolerance of the internal reference

±10 mV at 25∞C with a typical temperature coefficient of

25 ppm/

∞C and a maximum error over temperature of ±20 mV.

If the application requires a reference with a tighter tolerance

or if the AD7891 needs to be used with a system reference, an

external reference can be connected to the REF OUT/REF IN

pin. The external reference overdrives the internal reference

and thus provides the reference source for the ADC. The refer-

ence input is buffered before being applied to the ADC and

the maximum input current is

±100 mA. Suitable reference for

the AD7891 include the AD580, the AD680, the AD780, and

the REF43 precision 2.5 V references.

Analog Input Section

The AD7891 is offered as two part types: the AD7891-1 where

each input can be configured to have a

±10 V or a ±5 V input

range, and the AD7891-2 where each input can be configured

to have a 0 V to +2.5 V, 0 V to +5 V, and

±2.5 V input range.

AD7891-1

Figure 5 shows the analog input section of the AD7891-1. Each

input can be configured for

±5 V or ±10 V operation. 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 voltage is applied to the

VINXA input. The VINXA and VINXB inputs are symmetrical and

fully interchangeable. Therefore, 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.

30k

VINXA

VINXB

AGND

MULTIPLEXER

AD7891-1

REF OUT/REF IN

TO ADC

REFERENCE CIRCUITRY

7.5k

30k

15k

2.5V

REFERENCE

Figure 5. AD7891-1 Analog Input Structure

The input resistance for the

±5 V range is typically 20 kW. For

the

±10 V input range, the input resistance is typically 34.3 kW.

The resistor input stage is followed by the multiplexer, which 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).

LSB size is given by the formula 1 LSB = FS/4096. Therefore, for

the

±5 V range, 1 LSB = 10 V/4096 = 2.44 mV. For the ±10 V

range, 1 LSB = 20 V/4096 = 4.88 mV. Output coding is deter-

mined by the FORMAT bit of the control register. The ideal

input/output code transitions are shown in Table I.

AD7891-2

Figure 6 shows the analog input section of the AD7891-2. Each

input can be configured for input ranges of 0 V to +5 V, 0 V to +2.5 V,

±2.5 V. For the 0 V to 5 V input range, the VINXB input is

tied to AGND and the input voltage is applied to the VINXA input.

For the 0 V to 2.5 V input range, the VINXA and VINXB inputs

are tied together and the input voltage is applied to both. For

the

±2.5 V input range, the VINXB input is tied to 2.5 V and

the input voltage is applied to the VINXA input. The 2.5 V source

must have a low output impedance. If the internal reference on

the AD7891 is used, it must be buffered before being applied to

VINXB. The VINXA and VINXB inputs are symmetrical and fully

interchangeable. Therefore, for ease of PCB layout on the 0 V to +5 V

±2.5 V range, the input voltage may be applied to the VINXB

input, while the VINXA input is tied to AGND or 2.5 V.

1.8k

VINXA

VINXB

AGND

MULTIPLEXER

AD7891-2

REF OUT/REF IN

TO ADC

REFERENCE

CIRCUITRY

1.8k

2.5V

REFERENCE

Figure 6. AD7891-2 Analog Input Structure

The input resistance for both the 0 V to +5 V and

±2.5 V ranges

is typically 3.6 k

W. When an input is configured for 0 V to 2.5 V

operation, the input is fed into the high impedance stage of the

track/hold amplifier via the multiplexer and the two 1.8 k

resistors in parallel.

The designed code transitions occur midway between successive

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

is given by the formula 1 LSB = FS/4096. Therefore, for the 0 V

to 5 V range, 1 LSB = 5 V/4096 = 1.22 mV, for the 0 V to 2.5 V

range, 1 LSB = 2.5 V/4096 = 0.61 mV, and for the

±2.5 V range,

1 LSB = 5 V/4096 = 1.22 mV. Output coding is determined by

the FORMAT bit in the control register. The ideal input/output

code transitions for the

±2.5 V range are shown in Table I. The

ideal input/output code transitions for the 0 V to 5 V range and

the 0 V to 2.5 V range are shown in Table II.

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