MCP3564 datasheet(33/108 Pages) MICROCHIP

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DS20006181C-page 33

MCP3561/2/4

5.1.1

BURNOUT CURRENT SOURCES

FOR SENSOR OPEN/SHORT

DETECTION

The ADC inputs, VIN-/VIN+, feature a selectable burn-

out current source, which enables open or short-circuit

detection, as well as biasing very low-current external

sensors. The bias current is sourced on the VIN+ pin of

the ADC (noninverting output of the analog multiplexer)

and sunk on the VIN- pin of the ADC (inverting output of

the analog multiplexer). Since the same current flows

at the VIN-/VIN+ pins of the ADC, it can sense the

impedance of an externally connected sensor that

would be connected between the selected inputs of the

multiplexer. When the sensor is in short circuit, the

ADC converts signals that are close to 0V. When the

sensor is an open circuit, the ADC converts signals that

are close to the AVDD voltage.

The current source is an independent peripheral of the

ADC. It does not need the ADC to be in Conversion

mode to be present. Once enabled, the source pro-

vides current even when the ADC is in Reset or ADC

Shutdown mode. The current source can be configured

at any time through programming the CS_SEL[1:0] bits

in the CONFIG0 register (see Table 5-2).

Since the amount of current selected can be very small,

it may be necessary to diminish the MCLK master clock

frequency to be able to reach the full desired accuracy

during conversions (the settling time of the input

structure, including the sensor, can be large if the

sensor is very resistive, which will limit the bandwidth of

the Sample-and-Hold input circuit).

The accuracy of the current sources is on the order of

magnitude of ±20% and not very well controlled

internally. However, the mismatch between sink and

source is typically around ±1%.

This relatively low accuracy on the current is generally

sufficient

for

open/short

detection

applications.

Figure 2-35 shows how the ADC output code varies

when the burnout current sources are enabled (with

Gain = 1x) and the input sensor impedance is swept

with a large dynamic range. This allows the use of the

ADC as an open/short-detection circuit, which is

practical when manufacturing complex remote sensor

systems.

5.1.2

INTERNAL TEMPERATURE

SENSOR

The device includes an on-board temperature sensor,

which is made of two typical P-N junction diodes biased

by fixed current sources (TEMP Diode P and M). The

TEMP Diode P has a current density of 4x of the TEMP

Diode M.

The difference in the current densities of the diodes yields

a voltage that is a function of the absolute temperature.

Once the ADC inputs (VIN-/VIN+) are connected to the

temperature sensor diodes (MUX[7:0] = 0xDE), the

ADC will see a VIN differential input that is the function

of the temperature. The transfer function of the

temperature sensor can be approximated by a linear

equation or a third-order equation for more accuracy.

When the internal temperature sensor is selected for

the MUX or SCAN input, the input sink/source current

source, controlled by the CS_SEL[1:0] bits (see

Section 5.1 “Analog Input Multiplexer”), is disabled

internally (even though the CS_SEL[1:0] bits are not

modified by the temperature sensor selection). In this

case, the input current source is replaced by a specific

internal current source that will only be sourced to the

diode temperature sensor (see Figure 5-1).

The bias current of the diodes is not calibrated internally

and can lead to a relatively large gain and offset error in

the transfer function of the temperature sensor. Typical

graphs showing the typical error in the temperature

measurement are provided in Section 2.0 “Typical

Performance Curves” (see Figure 2-32 for first-order

and Figure 2-33 for third-order fitting).

The accuracy can also be optimized by using proper

digital gain and offset error calibration schemes.

TABLE 5-2:

BURNOUT CURRENT

SOURCE SETTINGS

CS_SEL[1:0]

(Source/Sink)

Burnout Current

Amplitude

0µA

0.9 µA

3.7 µA

15 µA

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