REFERENCE INPUT
The voltage into the VREF input is not buffered and directly
drives the Capacitor Digital-to-Analog Converter (CDAC)
portion of the ADS7841. Typically, the input current is
13µA with a 2.5V reference. This value will vary by
microamps depending on the result of the conversion. The
reference current diminishes directly with both conversion
rate and reference voltage. As the current from the reference
is drawn on each bit decision, clocking the converter more
quickly during a given conversion period will not reduce
overall current drain from the reference.
The external reference sets the analog input range. The
ADS7841 will operate with a reference in the range of
100mV to +VCC. Keep in mind that the analog input is the
difference between the +IN input and the –IN input, see
Figure 2. For example, in the single-ended mode, a 1.25V
reference, and with the COM pin grounded, the selected input
channel (CH0 - CH3) will properly digitize a signal in the
range of 0V to 1.25V. If the COM pin is connected to 0.5V,
the input range on the selected channel is 0.5V to 1.75V.
There are several critical items concerning the reference input
and its wide voltage range. As the reference voltage is re-
duced, the analog voltage weight of each digital output code
is also reduced. This is often referred to as the LSB (least
significant bit) size and is equal to the reference voltage
divided by 4096. Any offset or gain error inherent in the ADC
will appear to increase, in terms of LSB size, as the reference
voltage is reduced. For example, if the offset of a given
converter is 2LSBs with a 2.5V reference, then it will typically
be 10LSBs with a 0.5V reference. In each case, the actual
offset of the device is the same, 1.22mV.
DIGITAL INTERFACE
Figure 3 shows the typical operation of the ADS7841’s
digital interface. This diagram assumes that the source of the
digital signals is a microcontroller or digital signal processor
with a basic serial interface (note that the digital inputs are
over-voltage tolerant up to 5.5V, regardless of +VCC). Each
communication between the processor and the converter
consists of eight clock cycles. One complete conversion can
be accomplished with three serial communications, for a
total of 24 clock cycles on the DCLK input.
Likewise, the noise or uncertainty of the digitized output will
increase with lower LSB size. With a reference voltage of
100mV, the LSB size is 24µV. This level is below the
internal noise of the device. As a result, the digital output
code will not be stable and vary around a mean value by a
number of LSBs. The distribution of output codes will be
gaussian and the noise can be reduced by simply averaging
consecutive conversion results or applying a digital filter.
The first eight clock cycles are used to provide the control
byte via the DIN pin. When the converter has enough
information about the following conversion to set the input
multiplexer appropriately, it enters the acquisition (sample)
mode. After three more clock cycles, the control byte is
complete and the converter enters the conversion mode. At
this point, the input sample-and-hold goes into the hold
mode. The next twelve clock cycles accomplish the actual
Analog-to-Digital conversion. A thirteenth clock cycle is
needed for the last bit of the conversion result. Three more
clock cycles are needed to complete the last byte (DOUT
will be LOW). These will be ignored by the converter.
With a lower reference voltage, care should be taken to
provide a clean layout including adequate bypassing, a clean
(low-noise, low-ripple) power supply, a low-noise reference,
and a low-noise input signal. Because the LSB size is lower,
the converter will also be more sensitive to nearby digital
signals and electromagnetic interference.
CS
tACQ
DCLK
DIN
1
8
1
8
1
8
SGL/
DIF
S
A2 A1 A0 MODE
Idle
PD1 PD0
Acquire
(START)
Conversion
Idle
BUSY
DOUT
11 10
(MSB)
9
8
7
6
5
4
3
2
1
0
Zero Filled...
(LSB)
FIGURE 3. Conversion Timing, 24-Clocks per Conversion, 8-Bit Bus Interface. No DCLK delay required with dedicated
serial port.
ADS7841
10
SBAS084B
TI [ TEXAS INSTRUMENTS ]
