Datasheet

AD7707
Rev. B | Page 46 of 52
CHART RECORDERS
Another area where both high and low level input channels are
usually found is in chart recorder applications. Circular chart
recorders generally have two requirements. The first utilizes the
low level input channels of the AD7707 to measure inputs from
thermocouples, RTDs, and pressure sensors. The second
requirement is to be able to measure dc input voltage ranges up
to ±10 V. The high level input channel is ideally suited to this
measurement because there is no external signal conditioning
required to accommodate these high level input signals.
ACCOMMODATING VARIOUS HIGH LEVEL INPUT
RANGES
The high level input channel, AIN3 can accommodate input
sig
nals 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 Σ-Δ 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 31, 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
5kΩ
30kΩ
15kΩ
5kΩ
AIN3
VBIAS
HICOM
AIN3(+)
AIN3(–)
30kΩ
MUX
08691-031
Figure 31. 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
ranges. Table 28 applies for operation with AV
DD
= 5 V, and
REF(+) − REF(−) = 2.5 V. Table 29 applies for operation with
AV
DD
= 3 V, and REF(+) − REF(−) = 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 or source current. Table 30 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 28. Configuration of AD7707 vs. Input Range on AIN3 (AV
DD
= 5 V, V
REF
= 2.5 V)
AIN3 Range VBIAS HICOM Gain Buffered/Unbuffered AIN Range
±10 V 2.5 V AGND 2 Buffered/Unbuffered 1.875 V ± 1.25 V
±5 V 2.5 V AGND 4 Buffered/Unbuffered 1.875 V ± 0.625 V
0 V to 10 V 2.5 V AGND 2 Buffered/Unbuffered
1.875 V to 3.125 V
0 V to 20 V AGND AGND 1 Buffered
0 V to 2.5 V
2.5 V AGND 1 Buffered
1.875 V to 4.375 V
−5 V to +10 V 2.5 V 2.5 V 2 Buffered/Unbuffered 2.5 V ± 0.9375 V
Table 29. Configuration of AD7707 vs. Input Range on AIN3 (AV
DD
= 3 V, V
REF
= 1.25 V)
AIN3 Range VBIAS HICOM Gain BUF/UNBUF AIN Range
±5 V 1.25 V AGND 2 Unbuffered 0.9375 V ± 0.625 V
0 V to 10 V 1.25 V AGND 1 Unbuffered 0.9375 V to 2.1875 V
−5 V to +10 V 1.25 V 2.5 V 1 Unbuffered
1.5625 V ± 0.9375 V
−7.5 V to +10 V 1.25 V 0 V 1 Unbuffered
0 V to 2.1875 V
±10 V 1.666 V AGND 1 Unbuffered 1.25 V ± 1.25 V
Table 30. Typical Input Current vs. Voltage on AIN3
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 μA −229 μA