AD7575
APPLICATION HINTS
1. NOISE: Both the input signal lead to AIN and the signal
return lead from AGND should be kept as short as possible to
minimize input-noise coupling. In applications where this is
not possible, either a shielded cable or a twisted pair transmis-
sion line between source and ADC is recommended. Also,
since any potential difference in grounds between the signal
source and ADC appears as an error voltage in series with the
input signal, attention should be paid to reducing the ground
circuit impedance as much as possible. In general, the source
resistance should be kept below 2 kΩ. Larger values of source
resistance can cause undesired system noise pickup.
2. PROPER LAYOUT: Layout for a printed circuit board
should ensure that digital and analog lines are kept separated
as much as possible. In particular, care should be taken not to
run any digital track alongside an analog signal track. Both the
analog input and the reference input should be screened by
AGND. A single point analog ground separate from the logic
system ground, should be established at or near the AD7575.
This single point analog ground subsystem should be con-
nected to the digital system ground by a single-track connec-
tion only. Any reference bypass capacitors, analog input filter
capacitors or input signal shielding should be returned to the
analog ground point.
AD7575 WITH AD589 REFERENCE
The error analysis over temperature of ratiometric applications
is different from nonratiometric ones. Since the reference and
analog input voltage range are ratioed to each other, tempera-
ture variations in the reference are matched by variations in the
analog input range. Therefore, the AD589 contributes no addi-
tional errors over temperature to the system errors, and the
combined total unadjusted error specification for the AD589
and AD7575 is as per the total unadjusted error specification in
this data sheet.
With nonratiometric applications, however, the analog input
range stays the same if the reference varies and a full-scale error
is introduced. The amount by which the reference varies deter-
mines the amount of error introduced. The AD589 is graded on
temperature coefficient; therefore, selection of different grades
allows the user to tailor the amount of error introduced to suit
the system requirements. The reference voltage from the AD589
can lie between 1.2 V and 1.25 V. This reference voltage can be
adjusted for the desired full-scale voltage range using the circuit
outlined in Figure 19. For example, if an analog input voltage
range of 0 V to +2.46 V is required, the reference should be
adjusted to +1.23 V. Once the reference is adjusted to the de-
sired value at 25°C, the total error is as per the total unadjusted
error specification on the AD7575 specification pages. (To
reduce this still further, offset and full-scale errors of the
AD7575 can be adjusted out using the calibration procedure
outlined in this data sheet.)
+5V
10k *
6.8k
1k *
+5V
The AD7575 8-bit A/D converter features a total unadjusted
error specification over its entire operating temperature range.
This total unadjusted error includes all errors in the A/D con-
verter—offset, full scale and linearity. The one feature not pro-
vided on the AD7575 is a voltage reference. This section
discusses the use of the AD589 bandgap reference with the
AD7575, and gives the combined reference and ADC error
budget over the full operating temperature range. This allows
the user to compare the combined AD589/AD7575 errors to
ADCs whose specifications include on-chip references.
Two distinct application areas exist. The first is where the refer-
ence voltage and the analog input voltage are derived from the
same source. In other words, if the reference voltage varies, the
analog input voltage range varies by a ratioed amount. In this
case, the user is not worried about the absolute value of the
reference voltage. The second case is where changes in the refer-
ence voltage are not matched by changes in the analog input
voltage range. Here, the absolute value of the reference voltage,
and its drift over temperature, are of prime importance. Both
applications are discussed below.
If the analog input range varies with the reference voltage, the
part is said to be operating ratiometrically. This is representative
of many applications. If the reference is on-chip, and the user
does not have access to it, it is not possible to get ratiometric
operation. Since the AD7575 uses an external reference, it can
be used in ratiometric applications. However, because the part is
specified with a reference of +1.23 V
±
5%, then the voltage
range for ratiometric operation is limited.
TLC271*
+
AD589
–
10k *
*ONLY REQUIRED IF IT IS NECESSARY TO ADJUST
THE ABSOLUTE VALUE OF REFERENCE VOLTAGE.
Figure 19. Reference Adjust Circuit
However, it is as the temperature varies from 25°C that the
AD589 starts to introduce errors. The typical temperature char-
acteristics of the AD589 are shown in Figure 20. The tempera-
ture coefficients (TCs) represent the slopes of the diagonals of
the error band from +25°C to T
MIN
and +25°C to T
MAX
. The
AD589 TC is specified in ppm/°C max and is offered in four
different grades.
–10–
REV. B
AD [ ANALOG DEVICES ]
