R

S

C

SAMPLE

S

W

C

PIN

D

1

D

2

AVDD

AINx

S isclosedduringsampling.

S isopenduringconversion.

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 VREF. 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, D1 and D2, 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 D1 turns on when AINx is greater than AVDD + 0.3 V and diode D2 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 CPIN is approximately 8 pF. Resistor RS represents the sampling switch on-state resistance plus the

input multiplexer on-state resistance. The total resistance is approximately 130

Ω. CSAMPLE 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

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