AD7170 Datasheet, PDF (11/16 Pages) Analog Devices – 12-Bit Low Power Sigma-Delta ADC

DATA OUTPUT CODING
The AD7170 uses offset binary coding. Therefore, a negative
full-scale voltage results in a code of 000...000, a zero differential
input voltage results in a code of 100...000, and a positive full-
scale input voltage results in a code of 111...111. The output
code for any analog input voltage can be represented as
Code = 2^{N – 1} × [(VINx/VREF) + 1]
where:
VINx is the analog input voltage.
N = 12 for the AD7170.
REFERENCE
The AD7170 has a fully differential input capability for the
channel. The common-mode range for these differential inputs
is GND to VDD. The reference input is unbuffered; therefore,
excessive R-C source impedances introduce gain errors. The
reference voltage REFIN (REFIN(+) − REFIN(−)) is VDD
nominal, but the AD7170 is functional with reference voltages
of 0.5 V to VDD. In applications where the excitation (voltage or
current) for the transducer on the analog input also drives the
reference voltage for the part, the effect of the low frequency
noise in the excitation source is removed because the application
is ratiometric. If the AD7170 is used in a nonratiometric
application, a low noise reference should be used.
Recommended 2.5 V reference voltage sources for the AD7170
include the ADR381 and ADR391, which are low noise, low
power references. Also note that the reference inputs provide
a high impedance, dynamic load. Because the input impedance
of each reference input is dynamic, resistor/capacitor combinations
on these inputs can cause dc gain errors, depending on the output
impedance of the source that is driving the reference inputs.
Reference voltage sources such as those recommended above
(the ADR391, for example) typically have low output
impedances and are, therefore, tolerant to decoupling capacitors
on REFIN(+) without introducing gain errors in the system.
Deriving the reference input voltage across an external resistor
means that the reference input sees a significant external source
impedance. External decoupling on the REFIN(±) pins is not
recommended in this type of circuit configuration.
DIGITAL INTERFACE
The serial interface of the AD7170 consists of two signals: SCLK
and DOUT/RDY. SCLK is the serial clock input for the device,
and data transfers occur with respect to the SCLK signal. The
AD7170
DOUT/RDY pin is dual purpose: it functions as a data ready
pin and as a data out pin. DOUT/RDY goes low when a new
data-word is available in the output register. A 24-bit word is
placed on the DOUT/RDY pin when sufficient SCLK pulses are
applied. This consists of a 12-bit conversion result followed by
four 0s to generate a 16-bit word. Following this, eight status
bits are output. Table 8 shows the functions of the status bits.
RDY: ready bit. This bit is set low to indicate that a conversion
is available.
0: This bit is set to 0.
ERR: This bit is set to 1 if an error occurred during the conver-
sion. An error occurs when the analog input is outside range.
ID1, ID0: ID bits. These bits indicate the ID number for the
AD7170. Bit ID1 is set to 1, and Bit ID0 is set to 0 for the
AD7170.
PAT2, PAT1, PAT0: status pattern bits. They are set to 101 by
default. When the user reads the data from the AD7170, a
pattern check can be performed. If the PAT2 to PAT0 bits are
different from their default values, the serial transfer from the
ADC was not performed correctly.
Table 8. Status Bits
RDY 0 ERR ID1 ID0 PAT2 PAT1 PAT0
DOUT/RDY is reset high when the conversion is read. If the
conversion is not read, DOUT/RDY goes high prior to the data
register update to indicate when not to read from the device.
This ensures that a read operation is not attempted while the
register is being updated. Each conversion can be read only
once. The data register is updated for every conversion. So,
when a conversion is complete, the serial interface is reset, and
the new conversion is placed in the data register. Therefore, the
user must ensure that the complete word is read before the next
conversion is complete.
When PDRST is low, the DOUT/RDY pin is tristated. When
PDRST is taken high, the internal clock requires 1 ms, approx-
imately, to power up. Following this, the ADC continuously
converts. The first conversion requires the complete settling
time (see Figure 4). DOUT/RDY goes high when PDRST is
taken high and returns low only when a conversion is available.
The ADC then converts continuously, subsequent conversions
being available at 125 Hz. Figure 3 shows the timing for a read
operation from the AD7170.
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