Tektronix Keithley AMM1 Analog Measurement Module Rev. B User manual
AMMI
Analog Measurement Module
The AMMl Analog Measurement Module combines two important Series 500 functions
into a single module: the AMMl performs analog signal conditioning and switching,
and A/D conversion. The analog section of the module provides signal selection and
programmable gain for both local and global analog signals connected to the Series 500.
After analog conditioning, signals are routed to the AID converter section of the
module for the analog-to-digital conversion process.
The AMMl has a total of eight local single-ended inputs with unity (xl) local gain. Input signals are applied through on-card screw terminals. Global conditioning consists of
a high-speed software-controlled gain amplifier with programmable xl, x2, x5, and x10
gain values. Since all analog inputs connected to the Series 500 pass through the global
circuitry, these gain values can be applied to any analog input in the system.
For A/D conversion, the AMMl utilizes a K&bit successive approximation converter that
provides fast, accurate measurement and conversion. A maximum conversion time of
only 25psec and a sample-and-hold acquisition time of 3~s allow sampling rates as high
as 35.7kHz. To maximize resolution, the AMMl has five A/D converter ranges (three
bipolar, two unipolar) that can be selected by on-card DIP switches.
The AMMl is designed to be used only in slot 1 of the system baseboard. To install the
module, first remove the baseboard top cover and install the module in slot 1 with the
component side facing the power supply.
CAUTION: Always turn off the system power before installing or removing modules.
To minimize the possibility of EM1 radiation, always operate the system with the top
cover in place and properly secured.
User-Configured Components
User-configured components for the AMMl include the input screw terminals and the
switches that control A/D converter ranges, as surnmarized in Table 1. For the locations
of these components, refer to Figure 1.
Table 1. AMMl User-Configured Components
Description Designation Function
Screw Terminals
DIP Switch Set SlOl A/D Converter range
All local input signals are applied to screw terminals, which are designed to accept
16-24 gage wire stripped 3116 of an inch.
Local inputs, channels O-7
Document Number: 500-910-01 F&v. B
AMMl-1
A/D CONVERTER
RANGE SELECTION
I
6
ii
D
AMMI LOCAL
INPUT TERMINALS
END
C’H7
‘i-IS
c’ti5
C’H4
dH3
di2
C-H 1
C’HO
END
PI77
\ /
PI78
Switch SlOl controls the input range of the A/D converter located on the module.
Available bipolar ranges include
unipolar ranges are 0 to +lOV and 0 to +5V.
Connection
Local input signals for channels 0 through 7 of the AMMl are applied to screw ter-
minals located at the back edge of the board. The channel numbers are marked on the
board and are shown in Figure 1. Typical connections for channel 0 are shown in
Figure 2. Note that the high side of the input signal is applied to the channel 0 terminal, and the low side of the signal is connected to module ground.
-10 to +lOV, -5 to +5V, and -2.5 to +2.5V. The
0
0
0
0
0
0
0
0
I
Figure 2. Typical Connection (Channel 0 Shown)
CAUTION: AMMl inputs are non-isolated, meaning that one side of the input is connected to power line ground. Any signal connected to the AMMl must also be
referenced to power line ground, or module or system damage may occur. Also note
the inaccuracies on other channels may result.
In many situations, shielded cable may be required to minimize EM1 radiation, or to
keep noise to a minimum. If shielded cable is used, connect the shield to ground only,
and do not use the shield as a signal carrying lead. Usually, a module ground terminal
should be used, but in some cases better results may be obtained by using one of the
baseboard ground posts instead. Use the configuration that results in the lowest noise.
For shielding to be effective, the shield must contain both high and low signal wires,
and must not carry any other signals. If a number of AMMl signal input lines are
shielded, all shields should be connected to the same ground terminal.
AMMl-3
A/D Converter Range Selection
As shipped, the Ah&I1 is set up for the i-1OV range, but the module may be recon-
figured to one of four other ranges by setting the five DIP switches located on SlOl to
the correct positions, as summarized in Table 2. To set the A/D converter to a specific
range, first turn off system power and then set the switches to the correct positions,
either open (off) or closed (on). For example, for the 0 to +5V range, switches 1, 3 and
4 should be closed (on), and switches 2 and 5 should be open (off).
NOTE: The module must be recalibrated if the range is changed. Turn to the Calibration Section of this chapter for AMMl calibration information.
Table 2. 901 Settings for the A/D Converter Ranges
DIP Switches
Input Range 1
-10 to +lov*
-5 to +5v Open Open Closed Open Closed
-2.5 to +2.5V Open
0 to +5v
0 to +lov
*Factory default value
NOTE: A/D converter must be recalibrated if range is changed.
Open Closed Open Open Closed
Closed Open
Closed Open Closed Open Open
2
Open Closed Closed Closed
3
Closed Closed Open
4 5
Signal Conditioning
A simplified block diagram of the AMMl is shown in Figure 3. The module is divided
into four general sections: a local multiplexer, a global multiplexer, a programmable gain
amplifier (PGA) and a XI-bit AID converter.
Local input signals from channels 0 to 7 are applied to the local multiplexer for selection. At any given time, only one channel will be selected, as determined by the
SELECT CHANNEL command (covered later in this section). The signal from the
selected channel is then routed to the global multiplexer for further signal selection and
conditioning.
The global multiplexer selects a single signal from among the 10 slots in the signal. In
this manner, signals from any of the Xl slots can be selected by software. The global
multiplexer is controlled by the SELECT SLOT command, discussed later in this
section.
After the signal is selected, the PGA applies software-selectable gains of xl, x2, x5, or
x10. When this signal conditioning process is complete, the signal is routed to the l2-bit
AID converter for digitization. After the conversion process, digital data representing
the applied signal travels via the baseboard and interface card to the host computer.
AMMl-4
INPUTS
FROM OTHER
AIM MODULES
(9 MAXIMUM)
AMMI
LOCAL
INPUTS (8)
1.
\‘.
\
\
r
\
/
\
\-
GLOBAL
- MUX
,
xl, x2, x5OR x10
SOFTWARE
CONTROLLED GAIN
DIGITAL
INFORMATION
TO COMPUTER
Input Filtering
Noise introduced into the input signal can corrupt the accuracy of the measurement.
Such noise will usually be seen as an unsteady reading that jumps around, or, in some
cases, as a constant offset. In the former case, the presence of noise will usually be
quite obvious, but its effects may not be noticeable in the steady-state offset situation.
Regardless of the type of noise, however, such unwanted signals can degrade measurement accuracy considerably if enough of the unwanted signal is present.
Frequently, noise is introduced into the signal from 50 or 6OHz power sources. In many
cases, such noise can be attenuated by shielding the input signal lines, as discussed
earlier. In more difficult situations, however, it may be necessary to filter the input
signal to achieve the necessary noise reduction.
When noise is a problem, a single-pole low-pass filter like the one shown in Fiie 4
can be conntected between the input signal and the corresponding AMMl channel.
Note that the filter is made up of a single capacitor and resistor with the capacitor connected between the AMMl channel input terminal and the module
The resistor is then placed in series with the high input signal lead.
ground terminal.
1
27iRC
TO AMMl INPUT
7
FROM SIGNAL C
f
-3dB = -
T
Figure 4. Input Filtering
A common yardstick for a simple filter like the one in Figure 4 is the -3dB or halfpower point, which is given as follows:
1
f-J, = -
2nRC
AMMl-6
where f is in Hz, C is in farads, and R is in ohms. Above this frequency, filter response
will roll off (decrease) at a rate of -2OdB per decade. Thus, each time the frequency increases by a factor of lo, the filter output voltage decreases by a factor of 10 (-20dB).
Although such filtering can quiet down a noisy signal, there is a trade-off in the form
of increased response time. This response time may be important in the case of a rapidly changing input signal. For the filter in Figure 4, the response time to 1% of final
value is 4.6RC, while the response time to 0.1% and 0.01% of final value are 6.9RC and
9.2RC, respectively.
As an example, assume that 10 counts of 6OHz noise is present in the input signal. To
reduce the noise to one count, an attenuation factor of I.0 (-20dB) at 6OHz will be
necessary. Thus, the filter should have a -3dB point of 6Hz.
To determine the relative RC values, the above equations can be rearrange to solve for
either R or C. If we wish to choose a nominal capacitor value and then solve for the
resistance, we have:
1
R=
PnCf+fB
Choosing a nominal value of 2$ for C, the necessary resistance is:
1
R=
2n(2
x 10-6)x 6Hz
I? = 13.263k3
The resulting response times with these R and C values would be:
t(l%) = 4.6RC = 122ms
t(O.l%) = 6.9RC = 183ms
t(O.Ol%) = 9.2RC = 244ms
Note that there are a number of RC values that can be used in a given situation. To
minimize the effects of the series resistance, however, it is recommended that the value
of R be kept under 20kQ.
Current-to-Voltage Conversion
AMMl local inputs are designed to accept voltages in the range of flOV. Thus, the
AMMl can be directly connected to many signal sources. Some transducers and instrumentation, however, provide current outputs that must be converted into voltages in
order to be measured through an AMMl input channel.
When connecting current inputs to the AMMl, a resistor should be installed across the
input to make the necessary current-to-voltage conversion. One end of the resistor
should be connected to the channel input terminals and the other end of the resistor
should be connected to module ground.
AMMl-7
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