Mindray Passport 5-Lead, Service Manual

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Block Diagrams Theory of Operation
2 - 44 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
The output of the 8-bit D/A converter is multiplexed to two outputs: the external analog output
and the volume control voltage. The multiplexing is controlled by two sample and hold
control signals: ANEN* and VLEN*, controlled by port A bits 4 and 5 of the CPU
respectively. A zero in those bits will sample the voltage from the D/A converter and a one
will hold that voltage at the sample and hold.
Analog Output
The external analog output is used to output the ECG waveform. It is biased to be a bipolar
signal (+/-5V) by U36, U33, and R16/C78 bias network. A 00H code would output -5V
and an FFH code would correspond to +5V. The update rate should be 286Hz. The 10-bit
ECG sample from the front end should be scaled down to 8-bits (truncating the lower order
two bits) and output directly to the D/A. In order for the voltage at U34-6 to reach the 8 bit
lsb accuracy of the D/A, the switch should be held closed for at least 20 usecs. No other
writes to the D/A should occur within 20 usecs. Since the ECG is updated at a rate of 286
Hz (3.5 ms), there should be no problem from this half of the converter.
An analog simulation of the circuitry shows that it shall meet the specification of .5 to 40 Hz
@ -3db.
Speaker Driver
The volume voltage is unipolar. A 00H code would be lowest volume (silence) and FFH
would be highest volume. The loudness perception is not necessarily linear and some trial
and error may be needed to yield the corresponding sound levels desired for presets.
The digital switch for the speaker driver circuitry shall be held closed for at least 912 usecs.
in order to get the accuracy as discussed earlier.
The operation of the sample and hold should be as follows:
• Turn off both sample and holds (ANEN* and VLEN* high)
• Output volume value to D/A converter
• Turn on volume S/H (VLEN* low)
• Wait at least 912 usec
• Turn off volume S/H (VLEN* high)
• Output ECG value to D/A converter
• Turn on analog output S/H (ANEN* low)
• Wait at least 20 usec until starting again (S/H can be left on)
The EXEN* signal, controlled by port B bit 0, controls whether an exponential decay exists
on the volume signal (0 to enable decay). This mode is used only in beeps where an
exponential decay is desired. In this mode, the volume signal is enabled only at the start of
the beep and the volume will be allowed to decay by not updating the sample and hold.