Tektronix Keithley AOM2 High-Resolution Analog Output Module User manual

AOM2

High-Resolution Analog Output Module

The AOM2 High-Resolution Analog Output Module is available in two versions. The
AOM211 has one channel, while the AOM2/2 has two channels of analog output. Each
channel has a separate D/A converter. Unipolar and bipolar ranges are switch-selected
independently for each channel.

The converters provide true X-bit resolution with a maximum nonlinearity of stO.OOl5%
and low temperature coefficients. The AOM2 module also supports the system strobe
feature, which allows any number of output channels to be updated simultaneously.

Signal connections are made directly to screw terminals located on the module itself.

The AOM2 module may be placed in any slot in the baseboard. To install the module,
first turn off power and remove the top cover. Insert the module into the desired slot
with the component side facing the power supply. Place the AOM2 as far away from
the power supply as possible.

the possiblity of EMI radiation, never operate the system with the top cover removed.

User-Configured Components

Each channel of the high resolution AOM2 module has a range selection switch, op­tional filter capacitor location, and screw terminal for signal connection. In addition,
channel 0 or channel 1 may be connected to the AN OUT baseboard path. See Table 1
and Figure 1 for a summary of user-configured components on AOM2.

The range-selection switch sets, Sl.01 and SlO2 (AOM2/2 only), can be set for two out­put ranges; *lOV and 0 to +lOV. Optional filter capacitors help preserve the converter’s
X-bit accuracy through the reduction of noise.

Screw Terminals for both converters are located on a single bank, with a common ter­minal for ground connection.

Jumpers select whether channels 0 or 1 are connected to the analog output signal path,
and allow shorting of the output resistor for each channel.

’ CAUTION: Always turn off the power when installing or removing modules. To avoid

Document Number: 500-930-01 Rev. C

AOM2-1

r

AOMZ-2

Figure 1. AOM212 Module Configuration (AOMPM is similar except it has

only one channel)

Table 1. Summary of User-Configured Component on the AOM2
Name Designation Function

Capacitor 108 Cl08
Capacitor 105

Jumper Il.01

C105”

WlOl
Jumper 103 wlo3
Jumper 102 wlo2*

Screw Terminals J146
Switch Set 102 slo2
Switch Set 101 s101*

Optional noise filter channel 0
Optional noise

filter

channel 1
Select AN OUT signal (channel 0 or channel 1)
Short channel 0 output resistor
Short channel 1 output resistor
Output connection for channels 0 and 1
Analog output range channel 0
Analog output range channel 1

*AOM2/2 only.

Connections
Connecting terminals for the AOM2 module are marked on the module board. Typical

connections are shown in Figure 2. The use of shielded cable is recommended to
minimize the possibility of EMI radiation. Connect the shield to module ground, but
leave the other end disconnected. Do not use the shield as a signal-carrying lead.

LOAD

Figure 2. Typical AOM2 Connections (Channel 0 shown)

Range Switches
Each D/A converter on the AOM2 module has a switch set that controls the output

range. Each of these switch sets has two DIP switches that must be set to configure the
output for the desired range. Table 2 summarizes the positions for the DIP switches on
each set (these positions apply to both sets). Note that the module must be recalibrated
if the range is changed; see the Service section of this manual for details.

AOM2-3

Table 2. DIP Switch settings for Switch Sets SlOl and S102
Outuut Range 1 2

-10 to +lov*

oto +lov

*Factory default settings
Note: Switch settings are the same for both switch sets.

Preserving X-Bit Accuracy

With a &bit D/A converter, the value of a single voltage step is extremely small:

O.OOOl525V when the 0 to +lOV range is used. Electrical noise or temperature drift can
easily obscure such a small voltage change, reducing accuracy dramatically. The follow­ing sections suggest measures to maintain the accuracy of the converter.

Noise

Signal changes at the microvolt level are extremely difficult to transmit over wires even
a few inches long. Therefore, good signal connection practices should always be observ­ed. The length of connecting wires should be minimized, and these wires should never
run alongside AC power lines. In addition, shielded cable should always be used, with
the shielding connected to the ground terminal of the module.

If noise remains a problem, output filters for each channel can be installed on the
module itself.

Open
Closed

Closed
Open

Cl08 provides filtering for channel 0, while Cl05 is for channel 1. The required
capacitor value for a given bandwidth can be calculated as follows:

1

=-

C

P?rRf

Where C is in farads, f is in hertz and R is in ohms. Note that large capacitor values,
while providing greater noise rejection, will reduce output bandwidth accordingly. For
example, assume that the bandwidth is to be restricted to 3OHz. The required capacitor
value is (with a channel output resistance of lkn):

1

c=

6283(30)

C = 5pF

Note: Do not install filter capacitors with output resistors jumpered.

Analog output
A direct connection between the output of channel 0 or channel 1 and the AIM1 is

made through the baseboard connector’s AN OD’T signal path. An on-card Berg jumper

AOM2-4

(WlOl) is provided, so the connection can be defeated if noise from this source is
experienced. To connect channel 0 to the AN OUT path, place the WlOl jumper on the
middle two pins. To connect channel J, place the jumper on the left two pins. Place the
jumper on the right two pins to leave both channels disconnected.

Temperature Drift
D/A converters, like many electrical components, are sensitive to temperature change.

The AOM2 converters are designed with extremely low temperature coefficients (low
sensitivity to temperature changes); however, an attempt should be made to maintain
the operating environment near room temperature.

The converter has been calibrated to be accurate when operating. Because the
temperature of the device is higher when it is operating than when it is not in use, no

critical operations should be performed until the system has been on for at least five
minutes.

Output Loading

The source resistance of each output is lkQ with the output jumper removed. With this

configuration, a load resistance of IMQ will result in a loading error of only 0.1%. This
error may be compensated for by adjusting channel calibration as described in the ser-

vicing section of this manual. Note that with the resistor in the circuit, accuracy

decreases as the load resistance is decreased.

CAUTION: Placing the jumper across the output resistor eliminates output overload
protection. The module can be easily damaged with the jumper in place.

0

FROM D/A

CONVERTER

A

1ki-l

JUMPER

Figure 3. AOM2 Output Stage

AOM2-5