ADS1216 datasheet(12/28 Pages) TI

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ADS1216

SBAS171B

OVERVIEW

INPUT MULTIPLEXER

The input multiplexer provides for any combination of

differential inputs to be selected on any of the input chan-

nels, as shown in Figure 1. If channel 1 is selected as the

positive differential input channel, any other channel can be

selected as the negative differential input channel. With this

method, it is possible to have up to eight fully differential

input channels.

In addition, current sources are supplied that will source or

sink current to detect open or short circuits on the pins.

BURNOUT CURRENT SOURCES

When the Burnout bit is set in the ACR configuration register,

two current sources are enabled. The current source on the

positive input channel sources approximately 2

µA of current.

The current source on the negative input channel sinks ap-

proximately 2

µA. This allows for the detection of an open

circuit (full-scale reading) or short circuit (0V differential

reading) on the selected input differential pair.

INPUT BUFFER

The input impedance of the ADS1216 without the buffer

is 5M

Ω/PGA. With the buffer enabled, the input voltage range

is reduced and the analog power-supply current is higher. The

buffer is controlled by ANDing the state of the buffer pin with

the state of the BUFFER bit in the ACR register.

IDAC1 AND IDAC2

The ADS1216 has two 8-bit current output DACs that can be

controlled independently. The output current is set with

RDAC, the range select bits in the ACR register, and the 8-bit

digital value in the IDAC register. The output current

= VREF/(8 • RDAC)(2RANGE–1)(DAC CODE). With VREFOUT

= 2.5V and RDAC = 150kΩ, the full-scale output can be

selected to be 0.5, 1, or 2mA. The compliance voltage range

is 0 to within 1V of AVDD. When the internal voltage

reference of the ADS1216 is used, it is the reference for the

IDAC. An external reference may be used for the IDACs by

disabling the internal reference and tying the external refer-

ence input to the VREFOUT pin.

PGA

The Programmable Gain Amplifier (PGA) can be set to gains

of 1, 2, 4, 8, 16, 32, 64, or 128. Using the PGA can actually

improve the effective resolution of the A/D converter. For

instance, with a PGA of 1 on a 5V full-scale range, the A/D

converter can resolve to 1

µV. With a PGA of 128 on a 40mV

full-scale range, the A/D converter can resolve to 75nV. With

a PGA of 1 on a 5V full-scale range, it would require a 26-bit

A/D converter to resolve 76nV.

PGA OFFSET DAC

The input to the PGA can be shifted by half the full-scale input

range of the PGA by using the ODAC register. The ODAC

(Offset DAC) register is an 8-bit value; the MSB is the sign

and the seven LSBs provide the magnitude of the offset. Using

the ODAC does not reduce the performance of the A/D.

MODULATOR

The modulator is a single-loop second-order system. The

modulator runs at a clock speed (fMOD) that is derived from

the external clock (fOSC). The frequency division is deter-

mined by the SPEED bit in the setup register.

SPEED BIT

fMOD

0fOSC /128

1fOSC/ 256

IN3

IN4

IN5

IN6

IN0

IN1

IN2

IN7

INCOM

Burnout Current Source On

IDAC1

AGND

FIGURE 1. Input Multiplexer Configuration.

TEMPERATURE SENSOR

An on-chip diode provides temperature sensing capability.

When the configuration register for the input MUX is set to

all 1s, the diode is connected to the input of the A/D

converter. All other channels are open. The anode of the

diode is connected to the positive input of the A/D converter,

and the cathode of the diode is connected to negative input

of the A/D converter. The output of IDAC1 is connected to

the anode to bias the diode and the cathode of the diode is

also connected to ground to complete the circuit.

In this mode, the output of IDAC1 is also connected to the

output pin, so some current may flow into an external load

from IDAC1, rather than the diode.

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