Datasheet

R
S
C
SAMPLE
S
W
C
PIN
D
1
D
2
AVDD
AINx
S isclosedduringsampling.
S isopenduringconversion.
W
W
ADS8028
www.ti.com SBAS549B –MAY 2011–REVISED MARCH 2012
OVERVIEW
The ADS8028 is a 12-bit successive-approximation register (SAR) analog-to-digital converter (ADC) that can
support throughput rates up to 1 MSPS. The device features a 2.5-V internal reference, but can also function
with an external reference source. The analog input range is 0 V to V
REF
. The device supports eight single-ended
analog inputs and integrates an internal temperature sensor. A nine-channel (eight analog inputs plus the internal
temperature sensor) internal multiplexer allows selection of multiple channels to be scanned sequentially.
Additionally, this scan sequence can be repeated indefinitely with minimal intervention.
The internal temperature sensor has a resolution of 0.25°C and measures the ADS8028 die temperature. To
measure the temperature of an external heat source, thermal resistance should be minimized between the heat
source and the ADS8028 thermal pad (refer to the Application Information section for more details).
The ADS8028 consumes only 17 mW of power at 1 MSPS and also provides hardware (PD) and software
(STANDBY) selectable low-power modes for optimal power usage.
The device provides an SPI-compatible serial interface that can operate up to 20 MHz over a wide supply range
(DVDD = 1.65 V to 5.25 V). The ADS8028 operates with one cycle latency, thus the conversion result performed
in one cycle can be read out in the subsequent cycle.
ANALOG INPUTS AND MULTIPLEXER
The ADS8028 has eight single-ended analog input channels (AIN0 to AIN7). Figure 31 shows an equivalent
circuit for each analog input pin. The two diodes, D
1
and D
2
, provide electrostatic discharge (ESD) protection for
the individual analog pins. These diodes can conduct approximately 10 mA of current without causing irreversible
damage to the device. Diode D
1
turns on when AINx is greater than AVDD + 0.3 V and diode D
2
turns on when
AINx is less than AGND – 0.3 V. Therefore, care must be taken to always ensure that Equation 1 is met.
AGND – 0.3 V < AINx < AVDD + 0.3 V (1)
Figure 31. Equivalent Analog Input Circuit
Capacitor C
PIN
is approximately 8 pF. Resistor R
S
represents the sampling switch on-state resistance plus the
input multiplexer on-state resistance. The total resistance is approximately 130 Ω. C
SAMPLE
is the ADC sampling
capacitor, typically 40 pF.
The ADS8028 contains a nine-channel input multiplexer that either allows one of the eight analog input channels
or the internal temperature sensor to be converted. Multiple channels can be converted in a predetermined
sequence; this sequence can be repeated indefinitely with appropriate Control Register settings (refer to the
Modes of Operation section for more details). On power-up, no channel is selected for conversion and SDO
returns all '1's. One write cycle must be executed to select the channels and start the conversion process.
In order to achieve specified signal-to-noise ratio (SNR) and total-harmonic-distortion (THD) performance,
especially at higher input frequencies, it is recommended to drive each analog input pin with a low impedance
source. An external amplifier can also be used to drive the input pins. A simple RC low-pass filter can be used on
the analog input pins to reduce the input signal bandwidth and remove the noise components at higher
frequencies (refer to the Application Information section for more details).
Copyright © 2011–2012, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): ADS8028