Global Mixed-mode Technology Inc.
G781
ADC and Multiplexer
Detailed Description
The ADC is an averaging type that integrates over a
60ms period (each channel, typical), with excellent
noise rejection.
The G781 is a temperature sensor designed to work in
conjunction with an external microcontroller (µC) or
other intelligence in thermostatic, process-control, or
monitoring applications. The µC is typically a power-
management or keyboard controller, generating
SMBus serial commands by “bit-banging” general-
purpose input-output (GPIO) pins or via a dedicated
SMBus interface block.
The multiplexer automatically steers bias currents
through the remote and local diodes, measures their
forward voltages, and computes their temperatures.
Both channels are automatically converted once the
conversion process has started, either in free-running
or single-shot mode. If one of the two channels is not
used, the device still performs both measurements,
and the user can simply ignore the results of the un-
used channel. If the remote diode channel is unused,
tie DXP to DXN rather than leaving the pins open.
Essentially an serial analog-to digital converter (ADC)
with a sophisticated front end, the G781 contains a
switched current source, a multiplexer, an ADC, an
SMBus interface, and associated control logic (Figure
1). Temperature data from the ADC is loaded into two
data registers, where it is automatically compared with
data previously stored in several over/under- tem-
perature alarm registers.
The worst-case DXP-DXN differential input voltage
range is 0.25V to 0.95V.
Excess resistance in series with the remote diode
causes about +0.6°C error per ohm. Likewise, 240µV
of offset voltage forced on DXP-DXN causes about
1°C error.
VCC
MUX
2
7
DXP
DXN
+
+
REMOTE
LOCAL
ADC
+
SMBUS
SMBDATA
SMBCLK
CONTROL
LOGIC
WRITE
READ
DIODE
FAULT
8
8
REMOTE TEMPERATURE
DATA REGISTER
COMMAND BYTE
LOCAL EMPERATURE
DATA REGISTER
8
8
11
11
(INDEX) REGISTER
HIGH-TEMPETATURE
HIGH-TEMPETATURE
THRESHOLD (LOCALT
STATUS BYTE
REGISTER
THRESHOLD (REMOTE
)
HIGH
)
)
HIGH
LOW-TEMPETATURE
THRESHOLD (REMOTE
LOW-TEMPETATURE
THRESHOLD (LOCAL T
CONFIGURATION
BYTE REGISTER
)
LOW
LOW
8
11
CONVERSION RATE
REGISTER
DIGITAL COMPARATOR
(REMOTE)
DIGITAL COMPARATOR
(LOCAL)
ALERT
ALERT RESPONSE
ADDRESS REGISTER
SELECTED VIA
S
Q
SLAVE ADD = 0001 100
R
THERM
THERM LIMIT AND
COMPARATOR
HYSTERESIS REGISTER
Figure 1. Functional Diagram
Ver: 1.0
Oct 02, 2002
TEL: 886-3-5788833
http://www.gmt.com.tw
4
ETC [ ETC ]
