Functional Description (Continued)
small (see Typical Performance Characteristics) to allow
direct 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
FIGURE 2. Reference Examples
THE ANALOG INPUTS
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.
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 “+”
input and then the “−” input is 1⁄2 of a clock period. The
change in the common-mode voltage during this short time
interval can cause conversion errors. For a sinusoidal
common-mode signal this error is:
where fCM is the frequency of the common-mode signal,
VPEAK is its peak voltage value
and fCLK is the A/D clock frequency.
For a 60Hz common-mode signal to generate a 1⁄4 LSB error
(≈5 mV) with the converter running at 250kHz, its peak value
would have to be 6.63V which would be larger than allowed
as it exceeds the maximum analog input limits.
Source resistance limitation is important with regard to the
DC leakage currents of the input multiplexer. While operating
near or at maximum speed bypass capacitors should not be
used if the source resistance is greater than 1kΩ. The
worst-case leakage current of ±1µA over temperature will
create a 1mV input error with a 1kΩ source resistance. An op
amp RC active low pass filter can provide both impedance
buffering and noise filtering should a high impedance signal
source be required.
The zero of the A/D does not require adjustment. If the
minimum analog input voltage value, VIN(MIN), is not ground
a zero offset can be done. The converter can be made to
output 0000 0000 digital code for this minimum input voltage
by biasing any VIN (−) input at this VIN(MIN) value. This
utilizes the differential mode operation of the A/D.
The zero error of the A/D converter relates to the location of
the first riser of the transfer function and can be measured by
grounding the VIN (−) input and applying a small magnitude
positive voltage to the VIN (+) input. Zero error is the differ-
ence between the actual DC input voltage which is neces-
sary to just cause an output digital code transition from 0000
0000 to 0000 0001 and the ideal 1⁄2 LSB value (1⁄2 LSB =
9.8mV for VREF = 5.000VDC).
A full-scale adjustment can be made by applying a differen-
tial input voltage which is 11⁄2 LSB down from the desired
analog full-scale voltage range and then adjusting the mag-
nitude of the VREFIN input for a digital output code which is
just changing from 1111 1110 to 1111 1111 (See figure en-
titled “Span Adjust; 0V ≤ VIN ≤ 3V”). This is possible only with
the ADC08134 and ADC08138. (The reference is internally
connected to VREFIN of the ADC08131).