AD7891AS-2 datasheet(12/20 Pages) AD

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AD7891

–12–

REV. A

CIRCUIT DESCRIPTION

Reference

The AD7891 contains a single reference pin labelled REF OUT/

REF IN, which 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 will result in gain errors in the transfer function

of the AD7891 and will add to the specified full scale errors on

the part. They will 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

µF 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 as the part has a FET switch in series with

the reference, resulting in a source impedance for this output of

2 k

Ω nominal. The tolerance of the internal reference is ±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, then an

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

pin. The external reference will overdrive the internal reference

and thus provide 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 µA. 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 V

INXB

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. 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.

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 kΩ. For

the

±10 V input range the input resistance is typically 34.3 kΩ.

The resistor input stage is followed by the multiplexer and this

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 = FS/4096. Thus

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

determined 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 or

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

INXB

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 V

INXB 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 inter-

nal reference on the AD7891 is used, then it must be buffered

before being applied to VINXB. The VINXA and VINXB inputs are

symmetrical and fully interchangeable. Thus for ease of PCB

layout on the 0 V to +5 V range or the

±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

Ω. 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

Ω

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 etc.).

LSB size is given by the formula 1 LSB = FS/4096. Thus 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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