Lawson Labs, Inc.
_____________________________________

3217 Phoenixville Pike
Malvern, PA  19355 USA

Phone:  610 725-8800
        800 321-5355
FAX:    610 725-9344
e-mail  lawsnlab@netaxs.com
www.lawsonlabs.com
        



Introduction

The Lawson Labs, Inc. Model 201-5 is a stacking expansion card to
provide analog output capability for the Model 201 24-bit Data 
Acquisition System. The digital-to-analog converter (DAC) card
is available in two- or five-channel versions. The standard range
is 0 to 5 VDC. The first two channels can be set at the factory for
+/- 5 or 0 to 10 volts or other custom ranges. Resolution is 
programmable up to a maximum of 16 bits. The Model 201-5 is intended 
for DC or very low frequency application. 


Because the precision reference on the Model 201 is used to generate 
the output voltages, long-term output stability is excellent. 
Voltages are set using the Model 201's expansion output commands. 
Adding a DAC card will not affect the other functions of the Model 
201. The Model 201, itself, does not need to be altered to accept the 
stacking expansion card.


Installation

1)  Unplug the power to the Model 201.

2)  Remove cover, if the standard enclosure is used. 
    
3)  Place notched thin open-cell foam on bottom of DAC card.

4)  Align 32-pin expansion connector and press card in place.

5)  If using standard enclosure, cut a 2 x 2 inch piece of the
    thick foam to fit along side the expansion board.

6)  If not using enclosure, install four #6 screws from under the 
    Model 201 and secure the DAC card in place.

7)  Connect wires for analog outputs. Wires can exit the standard
    case via the relief notches at the bottom sides, or via a hole 
    drilled for the purpose.  

8)  If using standard enclosure, place square thin foam on top of 
    DAC card and thick foam just behind the 15-pin analog connector.
    Replace cover.  


Operation

D201_DAC is an extension of Drivr201 with support for the Model 201-5
DAC card. Most of the new code is clearly marked at the end of the 
source listing. The few new or altered lines found in the body of the 
code are remarked with 201-5.

The DACs are initialized and set to their minimum output after sign-on
time. Thereafter, a "V" keystroke will allow any DAC to be set to
any voltage. The initialization does not need to be repeated unless
power is interrupted to the Modl 201. It is prudent to repeat the
initialization process whenever communication is temporarily lost. 
This precaution protects against the possibility of a partial reset 
which may occur under some brownout conditions.

You can change the TimeBaseCnt, FullScaleOutput OutputOffset, or 
NumDacs% variables in the source code. FullScaleOutput and 
OutputOffset should be set as follows:

Output Range            FullScaleOutput      OutputOffset
---------------------------------------------------------
 +/-5                      10                    5
 0 to 5                     5                    0
 0 to 10                   10                    0

Note: Consult factory for custom output ranges.

The driver software assumes that all active DACs have the same 
output range. 
 
Changing TimeBaseCnt allows adjusting the tradeoff between 
resolution, settling time and ripple. With TimeBaseCnt at 10000
resolution is 1 part in 10000, or 0.5 mv for the 5 volt range. 
Settling time to within 1% is just over 1 second. There will be 
about a millivolt of ripple at a frequency of 1 kHz. Doubling the  
TimeBaseCnt will double the resolution, but will also double 
the ripple and halve the ripple frequency.  

For applications where ripple is a concern, and long settling times 
can be tolerated, a heavier filter can be used which will reduce 
the ripple to below the noise floor. Contact the factory for more 
information concerning this option.

The DAC output buffers are not powered unless the Model 201 is awake.
You may see small voltages at the dac outputs during sleep mode, but
no drive current is available. When the Model 201 is first awakened, 
the DACs should be initialized, normally to their minimum output 
voltage. The DACs will remember their last settings if power was not 
cycled. 

Each analog output can drive at least 5 milliamps. For best stability, 
do not load a 5 volt output with less than 1 K Ohm, or a 10 volt output 
with less than 2 K Ohms. To prevent ground noise from interferring
with the Model 201, the Model 201-5's ground terminals are actively
buffered grounds. Therefore, the ground output terminals on the DAC
card have the same current limitations as the output voltages. If
significant ground currents are possible, reference the analog outputs
to Ground on the Model 201 itself, instead of on the DAC card. This
measure may introduce a few millivolts of offset voltage, but will not 
reduce stability or linearity unless excessive ground currents are 
flowing.

Note: For the 5-channel version, a second amplifier buffers the 
      additional ground terminal on the right end of the output 
      connector. Amplifier offsets may cause the two grounds to
      be at slightly different potentials.

Please refer to QBasic source listing (Dd201_DAC.BAS) for details on 
operating the DACs. 

FACTORY OPTIONS:

Two or five channels of analog output

Alternate output voltage ranges.

Alternate output filtering. 

Premium amplifiers for maximum temperature stability and reduced
offset voltages.

Summed outputs for increased resolution 

Alternate connector for outputs: 0.1 center single inline header

Some or all counter sections can be used as event counters.

Contact factory for option details.


TROUBLESHOOTING

If the output voltages change, but do not correspond to
the desired voltage, double check the analog output range 
constants at the start of the DAC initialization routine.

If voltages drift over more than a few millivolt range,
check the ground connections. Try removing all ground
wires from the Model 201-5 and connecting them instead on 
the Model 201 itself.

If the DAC's outputs rise when Model 201 is awakened, that
indicates that power was not cycled to the system. It is 
normal for the DACs to remember their settings while the 
Model 201 sleeps. 

If the DACs outputs drop unexpectedly to zero, the Model 
201 has probably gone to sleep.  

If the Model 201 will not sign on with the DAC board
installed, be sure there are no short circuits or miswires 
on the analog output connections. A miswire or short could 
cause a heavy load which could keep the Model 201's power 
supplies from turning on.




                      SPECIFICATIONS



      Model 201-5 Stacking Analog Output Card for Model 201


 
  ANALOG OUTPUT CHANNELS:      2 (Model 201-5-2) or 5 (Model 201-5-5)

  OUTPUT VOLTAGE RANGE:        Standard:   0 TO 5 Volts
                               Optional:   +/- 5, or 0 TO 10 Volts
                               other ranges available, consult factory 

  RESOLUTION:                  Programmable, up to 1 part in 65536
  
  ACCURACY:                    0.1% of Full Scale
                               (0.03% of Full Scale with software offset
                               correction)
                               premium low-offset versions available

  LINEARITY:                   0.01% of full scale at 25 degrees C 

  RIPPLE:                      5 volt range, 1 part in 10,000 resolution
                               1 milivolt RMS, typical 

  OUTPUT DRIVE CURRENT:        5 milliamps minimum
  
  TEMPERATURE COEFFICENT:      50 ppm per degree C, typical
                               (premium versions available)

  RESPONSE TIME CONSTANT:      0.3 Seconds         

  CURRENT REQUIREMENT:     5-Channel
                               14 Milliamps in sleep mode
                               18 Milliamps in operation 
                               (plus output load currents)

                           2-Channel
                               12 Milliamps in sleep mode
                               14 Milliamps in operation 
                               (plus output load currents)
                               
  SIZE:                        4.4 x 4.3 x 0.6 inches

  WEIGHT:                      3 ounces




