REV. A

AD7707

–37–

Accommodating Various High Level Input Ranges

The high level input channel, AIN3 can accommodate input signals from –11 V to +30 V on its input. This is achieved using

on-chip thin film resistors that map the signal on AIN3 into a usable range for the AD7707. The input structure is arranged

so that the sigma-delta converter sees the same impedance at its AIN(+) and AIN(–) inputs. The signal on the AIN3 input is

referenced to the HICOM input and the VBIAS signal is used to adjust the common-mode voltage at the modulator input.

In normal 5 V operation VBIAS is normally connected to 2.5 V and HICOM is connected to AGND. This arrangement

ensures that the voltages seen at the modulator input are within the common-mode range of the buffer.

The differential voltage, AIN, seen by the AD7707 when using the high level input channel is the difference between AIN3(+)

and AIN3(–) as shown in Figure 30 and must remain within the absolute common-mode range of the modulator.

AIN3(+) = (AIN3 + 6

× VBIAS+ V (HICOM))/8

AIN3(–) = 0.75

× VBIAS + 0.25 V (HICOM)

AIN = (AIN3 – V (HICOM))/8

AIN

5k

30k

15k

5k

AIN3

VBIAS

HICOM

AIN3(+)

AIN3(–)

30k

MUX

Figure 30. AIN3, High Level Input Channel Structure

The VBIAS and HICOM inputs are used to tailor the input range on the high level input channel to suit a variety of input

Table XIX

AIN3 RANGE

VBIAS

HICOM

GAIN

BUF/UNBUF

AIN RANGE

±10 V

2.5 V

AGND

2

BUF/UNBUF

1.875 V

± 1.25 V

±5 V

2.5 V

AGND

4

BUF/UNBUF

1.875 V

± 0.625 V

0 V to 10 V

2.5 V

AGND

2

BUF/UNBUF

1.875 V to 3.125 V

0 V to 20 V

AGND

AGND

1

UNBUF

0 V to 2.5 V

2.5 V

AGND

1

UNBUF

1.875 V to 4.375 V

–5 V to +10 V

2.5 V

2.5 V

2

BUF/UNBUF

2.5 V

± 0.9375 V

Table XX

AIN3 RANGE

VBIAS

HICOM

GAIN

BUF/UNBUF

AIN RANGE

±5 V

1.25 V

AGND

2

UNBUF

0.9375 V

± 0.625 V

0 V to 10 V

1.25 V

AGND

1

UNBUF

0.9375 V to 2.1875 V

–5 V to +10 V

1.25 V

2.5 V

1

UNBUF

1.5625 V

± 0.9375 V

–7.5 V to +10 V

1.25 V

0

1

UNBUF

0 V to 2.1875 V

±10 V

1.666 V

AGND

1

UNBUF

1.25 V

± 1.25 V

Typical Input Currents

When using the high level input channel, power dissipation is determined by the currents flowing in the AIN3, VBIAS and

HICOM inputs. The voltage level applied to these inputs determines whether the external source driving these inputs needs

to sink of source current. The following table shows the currents associated with the

±10 V input range. These inputs should

be driven from a low impedance source in all applications to prevent significant gain errors being introduced.

Table XXI

AIN3

VBIAS

HICOM

I (AIN3)

I (VBIAS)

I (HICOM)

–10 V

2.5 V

AGND

–354

µA

500

µA

–146

µA

0 V

2.5 V

AGND

–62

µA

250

µA

–188

µA

+10 V

2.5 V

AGND

229

µA

0

–229

µA