ADM1026
However, when scaling AIN0 to AIN5, it should be noted that
these inputs already have an on-chip attenuator, because their
primary function is to monitor SCSI termination voltages. This
attenuator loads any external attenuator. The input resistance of
the on-chip attenuator can be between 100 kΩ and 200 kΩ. For
this tolerance not to affect the accuracy, the output resistance
of the external attenuator should be very much lower than
this, that is, 1 kΩ in order to add not more than 1% to the
total unadjusted error (TUE). Alternatively, the input can be
buffered using an op amp.
Voltage Measurement Inputs
The internal structure for all the analog inputs is shown in
Figure 27. Each input circuit consists of an input protection
diode, an attenuator, plus a capacitor to form a first-order low-
pass filter that gives each voltage measurement input immunity
to high frequency noise. The −12 V input also has a resistor
connected to the on-chip reference to offset the negative voltage
range so that it is always positive and can be handled by the
ADC. This allows most popular power supply voltages to be
monitored directly by the ADM1026 without requiring any
additional resistor scaling.
Vfs −3.0
R1
R2
=
=
(
for AIN0 to AIN5
)
)
21.9kΩ
3.0
A
– A
IN5
IN0
(0V – 3V)
109.4kΩ
4.6pF
4.6pF
9.3pF
Vfs −2.5
R1
R2
(
for AIN6 to AIN9
2.5
52.5kΩ
A
– A
IN9
IN6
(0V – 2.5V)
Negative and bipolar input ranges can be accommodated by
using a positive reference voltage to offset the input voltage
range so that it is always positive. To monitor a negative input
voltage, an attenuator can be used as shown in Figure 29.
113.5kΩ
+12V
21kΩ
R2
V
REF
A
IN(0–9)
MUX
17.5kΩ
R1
V
IN
114.3kΩ
83.5kΩ
–12V
+5V
9.3pF
4.6pF
Figure 29. Scaling and Offsetting AIN0 − AIN9 for Negative Inputs
50kΩ
This offsets the negative voltage so that the ADC always sees a
positive voltage. R1 and R2 are chosen so that the ADC input
voltage is zero when the negative input voltage is at its
maximum (most negative) value, that is:
49.5kΩ
V
BAT
82.7kΩ
4.5pF
Vfs−
R1
*SEE TEXT
=
R2 VOS
21.9k
+V
CCP
This is a simple and low cost solution, but note the following:
109.4kΩ
18.5pF
•
Because the input signal is offset but not inverted, the input
range is transposed. An increase in the magnitude of the
negative voltage (going more negative) causes the input
voltage to fall and give a lower output code from the ADC.
Conversely, a decrease in the magnitude of the negative
voltage causes the ADC code to increase. The maximum
negative voltage corresponds to zero output from the ADC.
This means that the upper and lower limits are transposed.
For the ADC output to be full scale when the negative
voltage is zero, VOS must be greater than the full-scale
voltage of the ADC, because VOS is attenuated by R1 and
R2. If VOS is equal to or less than the full-scale voltage of
the ADC, the input range is bipolar but not necessarily
symmetrical.
Figure 27. Voltage Measurement Inputs
Setting Other Input Ranges
AIN0 to AIN9 can easily be scaled to voltages other than 2.5 V or
3 V. If the input voltage range is zero to some positive voltage, all
that is required is an input attenuator, as shown in Figure 28.
•
A
R1
IN(0–9)
R2
V
IN
This is a problem only if the ADC output must be full scale
when the negative voltage is zero.
Figure 28. Scaling AIN0 − AIN9
Rev. A | Page ±8 of 56
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