di Mauro Mariotti
1 Dicembre 2002
|
di Mauro Mariotti |
1 Dicembre 2002 |
|
|
A/D Converters
On a/d converters many pages could be written, and has been written with muntains of documents. I can only refer the reader to those documents. What is interesting to seismic amateurs is to record seismic signals of his sensors with the highest dynamic range.
A 16 bits a/d converter is recommended,
that allows converting a wide range of signals maintaining a high resolution
even for small signals.
For example, it's very important
to read variations of +/-30Volts output from a sensor and be able to read
small variations of 0.0001 Volts.
To do this high cost and sophisticated
a/d converters are needed. The boards described here try to solve these
problems.
One can start with an economic approach, at first, you can see this page where a simple 8 bit converter is described.
In this page two kinds of a/d boards are described that are specifically designed to record seismic signals. One is a 16 bit converter and one (new) is a 24 bits converter.
Both boards uses a RS232 interface
to be connected to a personal computer or to another suitable interface
or recorder.
The protocol is a proprietary binary
protocol. It is described in the documentation of the SEISMOWIN data logging
software.
24 bits Analog to digital conversion
board
This board is a low cost board,
compact and robust. It can be used for other applications too and is very
simple to use. It uses a serial port RS232.
The microcontroller has been programmed to execute many functions:
1) At first it maintains the time in year, month, day, hour, minutes and seconds. The time can be adjusted using the proper commands given by serial com port. The quartz is digitally trimmed at normal environemental temperature (20-25 Celsius degrees) this allows to have at maximum one second of error per week.
2) The card has embedded software for DCF77 clock signal decoding, so the internal clock remains synchronized with that TIME source. The DCF77 time can be automatically adjusted to local time using the proper time lag correction. Using the GPS / DCF converter this card can be accurately and constantly syncronized by the GPS timing.
3) The time is continuously generated by the card without needs of programming it at the frequency of one time stamp per second.
5) The microcontroller manages the sampling at fixed rate established by the user in frequency between 200SPS and 5SPS.
6) The speed settings are stored
in the internal EEPROM. This allows the card to continuously transmit samples
as soon power supply is applied to it. This allows a monodirectional flow
of data towards the acquisition equipment. This allows remote application
of the board.
Technical features of the 24bits
a/d card (preliminary)
Power supply:
8-12V dc (polarity reversal protected)
Dimensions:
165 x 65 x 20 mm
Firmware:
On flash of the ATMEL AT90S2313 microcontroller
Com Interface:
RS232 at 38400 baud (N,8,1)
Clock:
Embedded on chip with DCF77 decoder
(useable with GPS/DCF converter to have a GPS
timing)
Clock error:
1.6ppm at 22 celsius degrees
Jitter DCF77:
10..80mS (depending by the receiver)
SPS available:
200 / 100 / 50 / 25 / 20 / 10 / 5 (directly from the board)
Channels:
Three programmable (all channel samples at same data rate)
channels can be inhibited you can record 1 or 2 or 3 channel
Converter chip:
LT2440 a/d converter
Bits resolution:
24 bits
Voltage resolution:
298.02 nanoVolts (0.00000029802V)
Voltage range:
+/- 2.5V
Analog Inputs:
fully differential
Channel impedance:
> 100Kohm
Gain:
3,355,443 counts/Volts
Dynamic range:
144dB
Signal / noise range:
132dB @ 100SPS
Channel separation:
> 150db at 10Hz
Slew Time (3 channels):
0
This board can easily be connected
directly to the sensor without the usage of a preamplifier unit.
Because of the high impedance of
the input, a damping (shunt) resistor can be applied directly on the sensor
or in the input connector.
Shielded twisted pair signal cable
is recommended.
This card is fully assembled, tested,
and is available for sale from SARA snc.
16 bits Analog to digital conversion
board
This is a card with high performance,
low cost and small size. It can be used for other applications too and
is very simple to use. It uses the serial RS232 interface. On board, a
powerful microcontroller manage all functions of precise sampling and timing
for 4 channels. It can be programmed by simple binary commands that allow
you to setup it as desidered.
The microcontroller has been programmed to execute many functions:
1) At first it mantains the time in hour, minutes and seconds. The time can be adjusted using the proper commands given by serial comm port. The quartz is digitally trimmed at normal environement temperature (20-25 Celsius degrees) this allow to have at maximum one second of error per week.
2) The card embedded the software for DCF77 clock signal decoding, so the internal clock can remains syncronized with that TIME source. The DCF77 time can be automatically adjusted to local time using the proper time lag correction. Using the GPS / DCF converter this card can be better and more constatly syncronized by the GPS timing.
3) The time is continously generated by the card without needs of programming it at the frequency of one time stamp per second.
4) Toghether with the time stamp, the status of the two digital inputs available on board is provided.
5) The microcontroller manage the sampling of the channels mantaining at fix rate the skew time and providing the transmission of the samples requested each channel at the proper speed.
6) The speed settings are stored
in the internal EEPROM. This allow the card to continously transmit samples
as soon power supply is applied to it. This allows a monodirectional flow
of data toward the acquisition equipment. This allows remote application
of the card.
Technical features of the 16bits
a/d card
Power supply:
8-12V dc (anti-inversion protected)
Dimensions:
165 x 65 x 20 mm
Firmware:
On flash of the ATMEL AT90S2313 microcontroller
Comm Interface:
RS232 at 38400 baud (N,8,1)
Clock:
Embedded on chip with DCF77 decoder
(useable with GPS/DCF converter to have a GPS
timing)
Clock error:
1.6ppm at 22 celsius degrees
Jitter DCF77:
10..80mS (depending by the receiver)
SPS available:
100 / 50 / 33.3 / 25 / 20 / 10 / 5 / 2 / 1 (directly from the
card)
0.5 / 0.1 / 1 min (using SEISMOWIN)
Channels:
Four programmable. You can acquire any channels
(only one too) with his SPS each channel is
protected against extra voltage
Converter chip:
Burr-Brown ADS7825P
Bits resolution:
16 bits
Voltage resolution:
0,000305 volts
Voltage range:
+/- 10 volts
Channel impedance:
45.8 Kohm
Capacitance:
35pF
Gain:
3276.8 count/Volts
Dynamic range:
96dB
Signal / noise range:
84dB
Accuracy:
+/- 3 digits
Channel separation:
> 120db at 10Hz
Offset error drift:
+/- 2 ppm/°C
Absolute Offset error:
< +/-10 mV
Skew Time (4 channels):
<= 2.5mS
NOTICE!
It is available the 01.40 version of the firmware that allows to:
- Sample at real 100 SPS for 3 channel
and have a maximum of 325 SPS for all 4 channels
- The frequency of the crystal is
digitally trimmed to have a good precision during losing of the DCF77 signal
- The SPS informations are stored
on chip to have the possibility to link the card to a radio transmitter
at 14400 baud without send commands to the card.
With this firmware version a 11.0592 Mhz crystal is needed instead of 1.8432 Mhz of the previous firmware.
This card fully assembled and tested
it is available for sale from SARA snc.
Channels phase shift
The ADS7825 converter is an high
precision successive approximations converter with 4 multiplexed channels.
In seismic datalogging if a single
multiplexed converter is used a phase shift on sampled signal can become
a problem.
For this reason many electronic
designers prefers to use a single converter for each channel.
There are basicly three system to
samples signals with a multplexed converter: the simultaneous sampling,
the interval scanning and the continous scanning.
Ref. National Instruments Measurement
Hardware Tutorial
As you can see in the picture the
worst samlping method for seismic pourposes is the continous sampling.
The samples will be shifted in phase
from one channel to the next.
In the interval scanning the situation
is better, because (in the example) the two channels are sampled very near
in time and they will reproduce the waveform much better than continuous
scanning (even if always different from simultaneous sampling).
The digitizer proposed in this pages
acts in this way. The sampling between channels are performed so quick
(in some tents of microseconds) so that at number of sample the card can
generates the phase shift is reduced quite to zero.
You can see the effect on the following
chart. (Use the scroll bars to look all the chart)
In this chart a triangle shape wave
signal of about 3Hz is applied on three of the four channel available.
The channels has been sampled at
100SPS. You can see how the waveform are exactly the same for each channel.
Tha chart show 1 second of sampling
with the time division in tenths of seconds.
The time division is so magnified
that any phase shift would become evident if would exists.
In this test the appropriate interval
scanning method shows all is effectiveness.
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