For absolute accuracy, where the analog input varies be-
tween very specific voltage limits, the reference pin can be
biased with a time and temperature stable voltage source.
The LM385 and LM336 reference diodes are good low cur-
rent devices to use with these converters.
Functional Description (Continued)
3.0 REFERENCE CONSIDERATIONS
The voltage applied to the reference input to these convert-
ers defines the voltage span of the analog input (the differ-
ence between VIN(MAX) and VIN(MIN)) over which the 256
possible output codes apply. The devices can be used in ei-
ther ratiometric applications or in systems requiring absolute
accuracy. The reference pin must be connected to a voltage
source capable of driving the reference input resistance of
typically 3.5 kΩ. This pin is the top of a resistor divider string
used for the successive approximation conversion.
The maximum value of the reference is limited to the VCC
supply voltage. The minimum value, however, can be quite
small (see Typical Performance Characteristics) to allow di-
rect conversions of transducer outputs providing less than a
5V output span. Particular care must be taken with regard to
noise pickup, circuit layout and system error voltage sources
when operating with a reduced span due to the increased
sensitivity of the converter (1 LSB equals VREF/256).
In a ratiometric system, the analog input voltage is propor-
tional to the voltage used for the A/D reference. This voltage
is typically the system power supply, so the VREF pin can be
tied to VCC (done internally on the ADC0832). This technique
relaxes the stability requirements of the system reference as
the analog input and A/D reference move together maintain-
ing the same output code for a given input condition.
DS005583-57
DS005583-58
a) Ratiometric
b) Absolute with a reduced Span
FIGURE 2. Reference Examples
4.0 THE ANALOG INPUTS
VPEAK is its peak voltage value
The most important feature of these converters is that they
can be located right at the analog signal source and through
just a few wires can communicate with a controlling proces-
sor with a highly noise immune serial bit stream. This in itself
greatly minimizes circuitry to maintain analog signal accu-
racy which otherwise is most susceptible to noise pickup.
However, a few words are in order with regard to the analog
inputs should the input be noisy to begin with or possibly
riding on a large common-mode voltage.
and fCLK, is the A/D clock frequency.
For a 60 Hz common-mode signal to generate a 1
⁄ LSB error
(≈5 mV) with the converter running at 250 kHz, its peak value
would have to be 6.63V which would be larger than allowed
as it exceeds the maximum analog input limits.
4
Due to the sampling nature of the analog inputs short spikes
of current enter the “+” input and exit the “−” input at the
clock edges during the actual conversion. These currents
decay rapidly and do not cause errors as the internal com-
parator is strobed at the end of a clock period. Bypass ca-
pacitors at the inputs will average these currents and cause
an effective DC current to flow through the output resistance
of the analog signal source. Bypass capacitors should not be
used if the source resistance is greater than 1 kΩ.
The differential input of these converters actually reduces
the effects of common-mode input noise, a signal common
to both selected “+” and “−” inputs for a conversion (60 Hz is
most typical). The time interval between sampling the “+” in-
put and then the “−” input is 1
in the common-mode voltage during this short time interval
⁄
2
of a clock period. The change
This source resistance limitation is important with regard to
the DC leakage currents of input multiplexer as well. The
can cause conversion errors. For
common-mode signal this error is:
a
sinusoidal
±
worst-case leakage current of 1 µA over temperature will
create a 1 mV input error with a 1 kΩ source resistance. An
op amp RC active low pass filter can provide both imped-
ance buffering and noise filtering should a high impedance
signal source be required.
where fCM is the frequency of the common-mode signal,
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