Rainbow Electronics ADC1061 - Manual (Page 9)

2 0 Reference Considerations
The ADC1061 has two reference inputs These inputs
V
REF a
and V REF b
are fully differential and define the zero
to full-scale range of the input signal The reference inputs
can be connected to span the entire supply voltage range
(V
REF b e 0V V
REF a e V
CC
) for ratiometric applications
or they can be connected to different voltages (as long as
they are between ground and V
CC ) when other input spans
are required Reducing the overall V REF span to less than
5V increases the sensitivity of the converter (e g if V
REF e
2V then 1LSB e 1 953 mV) Note however that linearity
and offset errors become larger when lower reference volt-
ages are used See the Typical Performance Curves for
more information Reference voltages less than 2V are not
recommended
In most applications V
REF b
will simply be connected to
ground but it is often useful to have an input span that is
offset from ground This situation is easily accommodated
by the reference configuration used in the ADC1061
V
REF b
can be connected to a voltage other than ground as
long as the reference for this pin is capable of sinking cur-
rent If V
REF b
is connected to a voltage other than ground
bypass it with multiple capacitors
Since the resistance between the two reference inputs can
be as low as 400X the voltage source driving the reference
inputs should have low output impedance Any noise on ei-
ther reference input is a potential cause of conversion er-
rors so each of these pins must be supplied with a clean
low noise voltage source Each reference pin should nor-
mally be bypassed with a 10 mF tantalum and a 0 1 mF
ceramic capacitor More bypassing may be necessary in
some systems
The choice of reference voltage source will depend on the
requirements of the system In ratiometric data acquisition
systems with a power supply-referenced sensor the refer-
ence inputs are normally connected to V
CC and GND and
no reference other than the power supply is necessary In
absolute measurement systems requiring 10-bit accuracy a
reference with better than 0 1% accuracy will be necessary
3 0 The Analog Input
The ADC1061 samples the analog input voltage once every
conversion cycle When this happens the input is briefly
connected to an impedance approximately equal to 600X in
series with 35 pF Short-duration current spikes can there-
fore be observed at the analog input during normal opera-
tion These spikes are normal and do not degrade the con-
vertor’s performance
Note that large source impedances can slow the charging of
the sampling capacitors and degrade conversion accuracy
Therefore only signal sources with output impedances less
than 500X should be used if rated accuracy is to be
achieved at the minimum sample time If the sampling time
is increased the source impedance can be larger If a signal
source has a high output impedance its output should be
buffered with an operational amplifier The operational am-
plifier’s output should be well-behaved when driving a
switched 35 pF 600X load Any ringing or voltage shifts at
the op amp’s output during the sampling period can result in
conversion errors
Correct conversion results will be obtained for input volt-
ages greater than GND b 50 mV and less than V
a a
50 mV Do not allow the signal source to drive the analog
input pin more than 300 mV higher than AV
CC
and DV
CC
or
more than 300 mV lower than GND If the analog input pin is
forced beyond these voltages the current flowing through
the pin should be limited to 5 mA or less to avoid permanent
damage to the ADC1061
4 0 Inherent Sample-and-Hold
Because the ADC1061 samples the input signal once during
each conversion it is capable of measuring relatively fast
input signals without the help of an external sample-hold In
a conventional successive-approximation A D converter
regardless of speed the input signal must be stable to bet-
ter than
g LSB during each conversion cycle or signifi-
cant errors will result Consequently even for many relative-
ly slow input signals the signals must be externally sampled
and held constant during each conversion
The ADC1061 can perform accurate conversions of input
signals at frequencies from DC to greater than 160 kHz
without the need for external sampling circuitry
5 0 Power Supply Considerations
The ADC1061 is designed to operate from a a5V (nominal)
power supply There are two supply pins AV
CC and DV CC
These pins allow separate external bypass capacitors for
the analog and digital portions of the circuit To guarantee
accurate conversions the two supply pins should be con-
nected to the same voltage source and each should be
bypassed with a 0 1 mF ceramic capacitor in parallel with a
10 mF tantalum capacitor Depending on the circuit board
layout and other system considerations more bypassing
may be necessary
It is important to ensure that none of the ADC1061’s input or
output pins are ever driven to a voltage more than 300 mV
above AV
CC and DV CC or more than 300 mV below GND If
these voltage limits are exceeded the overdrive current into
or out of any pin on the ADC1061 must be limited to less
than 5 mA and no more than 20 mA of overdrive current (all
overdriven pins combined) should flow In systems with mul-
tiple power supplies this may require careful attention to
power supply sequencing The ADC1061’s power supply
pins should be at the proper voltage before signals are ap-
plied to any of the other pins
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