Datasheet

ADC_DOUT
¨
©
§
=
¸
¹
·
( )
15
2x
±
GAIN(VINP - VINN) x
VREFP - VREFN
LMP90077, LMP90078, LMP90079, LMP90080
SNAS521F –JULY 2011–REVISED MARCH 2013 www.ti.com
APPLICATIONS INFORMATION
QUICK START
This section shows step-by-step instructions to configure the LMP900xx to perform a simple DC reading from
CH0.
1. Apply V
A
= V
IO
= V
REFP1
= 5V, and ground V
REFN1
2. Apply V
INP
= ¾V
REF
and V
INN
= ¼V
REF
for CH0. Thus, set CH0 = V
IN
= V
INP
- V
INN
= ½V
REF
(CH0_INPUTCN
register)
3. Set gain = 1 (CH0_CONFIG: GAIN_SEL = 0x0)
4. Exclude the buffer from the signal path (CH0_CONFIG: BUF_EN = 1)
5. Set the background to BgcalMode2 (BGCALCN = 0x2)
6. Select V
REF1
(CH0_INPUTCN: V
REF_SEL
= 0)
7. To use the internal CLK, set CLK_EXT_DET = 1 and CLK_SEL = 0.
8. Follow the register read/write protocol (Figure 47) to capture ADC_DOUT from CH0.
CONNECTING THE SUPPLIES
V
A
and V
IO
Any ADC architecture is sensitive to spikes on the analog voltage, V
A
, digital input/output voltage, V
IO
, and
ground pins. These spikes may originate from switching power supplies, digital logic, high power devices, and
other sources. To diminish these spikes, the LMP900xx’s V
A
and V
IO
pins should be clean and well bypassed. A
0.1 µF ceramic bypass capacitor and a 1 µF tantalum capacitor should be used to bypass the LMP900xx
supplies, with the 0.1 µF capacitor placed as close to the LMP900xx as possible.
Since the LMP900xx has both external V
A
and V
IO
pins, the user has two options on how to connect these pins.
The first option is to tie V
A
and V
IO
together and power them with the same power supply. This is the most cost
effective way of powering the LMP900xx but is also the least ideal because noise from V
IO
can couple into V
A
and negatively affect performance. The second option involves powering V
A
and V
IO
with separate power
supplies. These supply voltages can have the same amplitude or they can be different.
V
REF
Operation with V
REF
below V
A
is also possible with slightly diminished performance. As V
REF
is reduced, the
range of acceptable analog input voltages is also reduced. Reducing the value of V
REF
also reduces the size of
the LSB. When the LSB size goes below the noise floor of the LMP900xx, the noise will span an increasing
number of codes and performance will degrade. For optimal performance, V
REF
should be the same as V
A
and
sourced with a clean source that is bypassed with a ceramic capacitor value of 0.1 µF and a tantalum capacitor
of 10 µF.
LMP900xx also allows ratiometric connection for noise immunity reasons. A ratiometric connection is when the
ADC’s V
REFP
and V
REFN
are used to excite the input device’s (i.e. a bridge sensor) voltage references. This type
of connection severely attenuates any V
REF
ripple seen the ADC output, and is thus strongly recommended.
ADC_DOUT CALCULATION
The output code of the LMP900xx can be calculated as:
Output Code (15)
ADC_DOUT is in 16−bit two's complement binary format. The largest positive value is 0x7FFF (or 32767 in
decimal), while the largest negative value is 0x8000 (or 32768 in decimal). In case of an over range the value is
automatically clamped to one of these two values.
Figure 58 shows the theoretical output code, ADC_DOUT, vs. analog input voltage, V
IN
, using the equation
above.
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