SS8017 datasheet(7/16 Pages) SSC

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A/D Conversion Sequence

If a Start command is written (or generated automati-

cally in the free-running auto-convert mode), both two

channels are converted, and the results of both meas-

urements are available after the end of conversion. A

BUSY status bit in the status byte shows that the de-

vice is actually performing a new conversion; however,

even if the ADC is busy, the results of the previous

conversion are always available.

Remote-Diode Selection

Temperature

accuracy

depends

having

good-quality, diode-connected small-signal transistor.

Accuracy has been experimentally verified for all of the

devices listed in Table 1. The 8017 can also directly

measure the die temperature of CPUs and other inte-

grated circuits having on-board temperature- sensing

diodes. The transistor must be a small-signal type with

a relatively high forward voltage; otherwise, the A/D

input voltage range can be violated. The forward voltage

must be greater than 0.25V at 10µA; check to ensure

this is true at the highest expected temperature. The

forward voltage must be less than 0.95V at 200A;

check to ensure this is true at the lowest expected

temperature. Large power transistors don't work at all.

Also, ensure that the base resistance is less than

100

Ω. Tight specifications for forward current gain (+50

to +150, for example) indicate that the manufacturer

has good process controls and that the devices have

consistent VBE characteristics.

Thermal Mass and Self-Heating

Thermal mass can seriously degrade the 8017's effec-

tive accuracy. The thermal time constant of the

SSOP-16 package is about 140sec in still air. For the

8017 junction temperature to settle to within +1°C after

a sudden +100°C change requires about five time con-

stants or 12 minutes. The use of smaller packages for

remote sensors, such as SOT23s, improves the situa-

tion. Take care to account for thermal gradients be-

tween the heat source and the sensor ,and ensure that

stray air current across the sensor package does not

interfere with measurement accuracy.

Table 1. Remote-Sensor Transistor Manufacturers

MANUFACTURER

MODEL NUMBER

Philips

PMBS 3904

Motorola (USA)

MMBT3904

National Semiconductor (USA)

MMBT3904

Note:Transistors must be diode-connected (base short

-ed to collector).

ADC Noise Filtering

The ADC is an integrating type with inherently good

noise rejection, especially of low-frequency signals

such as 60Hz/120Hz power-supply hum. Micro-power

operation places constraints on high-frequency noise

rejection; therefore, careful PC board layout and proper

external noise filtering are required for high- accuracy

remote measurements in electrically noisy environ-

ments.

High-frequency EMI is best filtered at DXP and DXN

with an external 2200pF capacitor. This value can be

increased to about 3300pF(max), including cable ca-

pacitance. Higher capacitance than 3300pF introduces

errors due to the rise time of the switched current

source.

Nearly all noise sources tested cause the ADC meas-

urements to be higher than the actual temperature,

typically by +1°C to 10°C, depending on the frequency

and amplitude (see Typical Operating Characteristics).

PC Board Layout

Place the 8017 as close as practical to the remote

diode. In a noisy environment, such as a computer

motherboard, this distance can be 4 in. to 8 in. (typical)

or more as long as the worst noise sources (such as

CRTs, clock generators, memory buses, and ISA/PCI

buses) are avoided.

Do not route the DXP -DXN lines next to the deflection

coils of a CRT. Also, do not route the traces across a

fast memory bus, which can easily introduce +30°C

error, even with good filtering, Otherwise, most noise

sources are fairly benign.

Route the DXP and DXN traces in parallel and in close

proximity to each other, away from any high-voltage

traces such as +12VDC. Leakage currents from PC

board contamination must be dealt with carefully, since

a 20M

Ω leakage path from DXP to ground causes

about +1°C error.

Route the 2 pairs of DXP1-DXN and DXP2-DXN traces

independently (Figure 2a). Connect the common DXN

as close as possible to the DXN pin on IC (Figure 2a).

Connect guard traces to GND on either side of the

DXP-DXN traces (Figure 2b). With guard traces in

place, routing near high-voltage traces is no longer an

issue.

Route through as few vias and crossunders as possible

to minimize copper/solder thermocouple effects.

When introducing a thermocouple, make sure that both

the DXP and the DXN paths have matching thermocou-

ples. In general, PC board- induced thermocouples are

not a serious problem, A copper-solder thermocouple

exhibits 3µV/°C, and it takes about 200µV of voltage

error at DXP -DXN to cause a +1°C measurement error.

SS8017

Rev.2.01 6/06/2003

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