ETC 160MK

Model 160MK Bridgesensor
Features
Description
n
Compact, complete and convenient to use
n
Easy access to all trim adjustments
n
Half Bridge applications made easy by Internal
Completion Resistors
n
On card Bridge Balance Trimpot eliminates
additional wiring for Three Wire applications
n
Changing Bridge supply voltage is easy using
on board trimpot with adjustment range from
+4 to +10 VDC
The CALEX 160MK Bridgesensor is a complete signal
conditioning system on a card designed expressly for either
half or full bridge transducers. The 160MK consists of a high
performance instrumentation amplifier, a user adjustable
active filter, high stability bridge supply and reference regulator
in a state-of-the-art hybrid circuit, which is mounted on a PC
board mounting kit containing all of the required external
circuitry, trimpots, etc., so that only point to point wiring need
be made to the inputs, outputs and power to have a complete
signal conditioning system up and running.
The mounting kit provides coarse and fine gain adjustment
trimpots along with input and output offset adjustments, DIP
switches for setting the bridge supply output and active low
pass filter cutoff frequency.
n
Bridge supply lead resistance effects can be
ignored with built-in remote sensing
n
Filter frequency can be changed with the flick
of a DIP switch
n
Worst Case design is simplified with fully specified
engineering specifications. No phone calls, no
letters, it’s all right here
Application of the 160MK is easy by following the detailed
applications information that is included with this data sheet
and full engineering specifications allow easy and complete
worst case analysis.
160MK Schematic
+15 VDC A
CMN B
KEY C
+SENSE D
-15 VDC E
)
BRIDGE SUPPLY F
-SENSE H
6.8 VDC REFERENCE J
EXT. OUTPUT OFFSET K
-INPUT L
+INPUT M
FILTERED OUTPUT N
AMPLIFIER OUTPUT P
BRIDGE COMPLETION R
BRIDGE BALANCE S
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FIGURE 1. Complete schematic of the 160MK Bridgesensor
Model 160MK Bridgesensor
Specifications
Conditions (Unless Noted): Ta = 25°C, Vs = ±15 VDC, G = 500 V/V
P a ra m e te r
Min im u m
Ty p ic a l
Ma x im u m
U n its
500
5000
V/V
Am p lifie r (1 )
Gai n Range
Adjustable (2)
w/External Set Resi stor
Gai n Equati on
100
2
Gai n Equati on Accuracy 2 < G < 1000 V/V
Rg = 80,000/(G-2)
3
Gai n Range w/Temperature:
w/Tri mpots
Ampli fi er alone
Nonli neari ty, ±10V Output Swi ng
%
75
15
0.002
Offset Voltage, Input and Output
0.005
%
75
4
V
10
25
5
10
C ommon Mode
Input Voltage:
C MR (6):
Range, Li near Response
Maxi mum
1 kHz bw, D C -60 Hz (7)
10 Hz bw, D C -60 Hz (7)
Input Noi se Voltage:
0.1 Hz - 10 Hz
10 Hz - 100 Hz
C urrent:
0.1 Hz - 10 Hz
10 Hz - 100 Hz
Rated Output:
Voltage, 2 kohm Load
C urrent
Load C apaci tance
Short C i rcui t
D ynami c Response (8):
Small Si gnal Bandwi dth
Amp Out Gai n Bandwi dth (9)
Low Pass Fi lter (10):
Number of Poles
D C Gai n (Pi n P to N)
Roll Off
B rid g e E x c ita tio n S u p p ly (11 )
w/Tri mpot
w/Ext Resi stor
110
110
± 10
±5
50
20
4G ohms | | 15 pF
±9.5
± 15
140
140
0.3
1
60
100
nA
pA/°C
nA
pA/°C
VDC
dB
µv P-P
pA P-P
1000
Indefi ni te
Adjustable
25
2
-2
40
VD C
mA
pF
kHz
MHz
V/V
dB/D ec
4
4
0
10
12
100
VDC
mA
0.1
0.2
%
Li ne Regulati on Vi n = 14.5 - 18 VD C
0.05
0.5
%/V
Stabi li ty (13):
0.05
0.2
40
0.1
80
%/24 Hrs
%/kHrs
ppm/°C
%
Load Regulati on I
L
= 0 - 100 mA
µV
µV/°C
µV/V
± 10
Input Bi as C urrent (4)
Vs. Temperature
Input Offset C urrent
Vs. Temperature
Input Impedance (5)
Output C urrent (12)
ppm/°C
1
30
1
±4 ±(100/G)
1
G = 2V/V
G = 1000V/V
At Other Gai ns, Max.
Vs. Power Supply Pi n K ti ed to Pi n B
Output Adjustment Range:
150
Adjustable to Zero
Warmup D ri ft (3)
Vs. Temperature:
Output Offset Adjust Range
ohms
Short Term
Long Term
Vs. Temperature
Warm-up D ri ft
Short C i rcui t Protecti on
)
8 Hours Mi ni mum
Output Noi se, 10 Hz - 1 kHz
200
Internal Reference Voltage (15)
6.46
6.80
µV P-P
7.14
VD C
H a lf B rid g e C o m p le tio n
Nomi nal Resi stance Value
20
kohms
Ini ti al Accuracy
1
%
Temperature Tracki ng
5
ppm/°C
Balance Adjustment Range
5
%
P o w e r R e q u ire m e n ts
C urrent (14)
E n v iro n m e n ta l
Ambi ent:
Rated Performance
Operati ng
± 12
Operati ng
Storage
-25
-40
± 15
± 18
± 12
VD C
mA
70
100
°C
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Voltage:
Model 160MK Bridgesensor
Notes:
(1) Specifications referred to the filter output (Pin N).
(2) Using on board coarse and fine gain adjust trimpots.
(3) Warm-up drift is specified as the input offset drift for the first
5 minutes after the application of power with G = 1000 V/V,
Bridge supply = 10V driving a 350 ohm bridge.
(4) Measured at 25°C Ambient with unit fully warmed up.
(5) Measured from -Input to +Input or input with respect to
ground.
(6) Specified with 1 kohm source impedance imbalance,
100 < Gain < 1000 V/V.
(7) Filter frequency set with DIP switches.
(8) Small signal response, switch or resistor/capacitor selectable,
see applications section.
(9) This is the instrumentation amplifier basic gain bandwidth
product, the filtered output will provide for a constant cutoff
frequency regardless of gain.
(10) The low pass filter cutoff frequency is adjustable to 10, 100
and 1000 Hz using the onboard DIP switches and from 1 Hz
to 10 KHz using external resistors and capacitors.
(11) Bridge supply must be operated with +Sense connected to
the Bridge Supply Pin and with -Sense connected to Common.
(12) The bridge supply maximum output current is a function of
input to output voltage differential and temperature. See
graphs and applications section for more information.
(13) Stability is defined after a 5 minute warm-up period and with
constant line, load and ambient temperature unless otherwise
specified.
(14) Quiescent current for amplifiers only, the current drawn from
the bridge supply must be added to the +15 VDC current
drain for total current draw.
(15) Referred to Pin J, this is a high impedance output and should
not be loaded greater than 50 MicroAmps. Buffer this reference
with an OP-AMP for greatest accuracy.
the mounting kit to disable the trimpots, then calculate the
required value for RG and solder it on the mounting kit in the
spot provided.
The gain equation accuracy is ±3 percent for gains from 2 to
1000 V/V.
RG =
80,000
G-2
ohms
Equation 1: User supplied resistor value required to set gain
with respect to Pin N, filtered output.
RG =
40,000
G-1
ohms
Equation 2: User supplied resistor value required to set gain
with respect to Pin P, amplifier direct output (this is an inverted
output). NOTE: If a fixed resistor is used for RG, then resistor
R6 should be removed from the 160MK to disable the gain
trimpots.
Example Resistor Values for Common Gains (to Filtered
Output):
Required Gain,
Filtered Output
10
100
Functional Description
The CALEX Model 160MK is a completely self contained
single channel signal conditioning system on a card. This
device offers the high performance and reliability of hybrid
circuitry with the completeness of a mounting kit containing all
trimpots and components needed for operation, all that needs
to be added is power and transducer inputs to get a conditioned
output suitable for driving A/D converters, panel meters,
indicators, or PC based controllers. A full schematic of the
160MK is shown in Figure 1.
RG
Value
10,000 ohms
816 ohms
333.33
241 ohms
500
160 ohms
1000
80.2 ohms
(Use for 3mV/V
Transducers)
(Use for 2mV/V
Transducers)
Note: A high stability, 5 ppm/°C metal film resistor should be selected for RG
for maximum performance.
The instrumentation amplifier also has a trimpot adjustment
for input offset voltage, this trimpot should be used to null the
instrumentation amplifier offset only. System offsets should
be adjusted out using the Bridge Balance or the Output Offset
feature (see applications section for more information) to
retain minimum offset drift of the instrumentation amplifier.
The 160MK inputs should be placed as close to the transducer
as possible. This will minimize any possible pickup of
electrostatic or electromagnetic noise into the very high
impedance inputs. See the applications section for more
information on shielding methods.
Instrumentation Amplifier
The heart of the 160MK is the high performance instrumentation
amplifier. This amplifier features low noise, low drift and high
accuracy along with trimpot adjustments for coarse/fine gain
and input offset voltage. The direct instrumentation amplifier
output is brought out to Pin P on the 160MK, this signal is
inverted ( Vo = Gain x ( - (+Input - -Input) ) with respect to the
input pin labeling but can be used where the full 25 MHz gain
bandwidth product is required such as for mechanical vibration
analysis. This output is also brought out to the test point AMP
OUT at the trimpot edge of the mounting kit. The trimpots allow
a gain adjustment range of 100 to 500 V/V with a coarse and
fine gain adjuster (clockwise rotation increases gain). A user
supplied resistor can be used in place of the trimpots (see
equations below) to get any gain from 2 to 5000V/V (referred
to filtered output). To use an external resistor remove R6 from
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)
Model 160MK Bridgesensor
Active Filter
BUILT IN LOW PASS FILTER FREQUENCY RESPONSE
The output of the instrumentation amplifier is internally
connected to the input of a 2 pole, inverting gain (-2V/V)active
filter. This filter has an adjustable filter cutoff frequency of 10,
100 and 1kHz by the use of on board DIP switches and can be
set to any frequency from 10 Hz to 10 kHz by the use of user
supplied resistors and capacitors. The filtered output is brought
out to Pin N and to test point FIL OUT at the trimpot end of the
board on the 160MK. Pin N is the standard output for most
strain gage and instrumentation applications, by using the
filtered output extraneous noise above the useful signal
frequency is removed at a rate of 40dB/decade above the filter
cutoff frequency allowing very precise and low noise
measurements to be made. Figure 2 details the DIP switch
settings and the equations required to set the filter cutoff to
any other frequency.
GAIN (db) PIN P - PIN N
10
SW3
10 Hz
ON
ON
100 Hz
1000 Hz
or User Select
C1 = 0.0022 µF
SW4
SW5
ON
ON
10Hz
10
1000
A location is provided for damping capacitors C12 and C13
on the 160MK to provide AC coupling and compensate for
)
-1
BRIDGE SUPPLY Vs. R EXC
240
-1
200
-1
-1
1000
R3 = 40,000/
Fc
100000
The bridge supply uses + and - sense connections to
compensate for any line drops that might be present when
using remote transducers, see the applications examples for
more information on properly using the + and - sense pins.
If remote sensing is not required connect +Sense (Pin D) to
Bridge Supply (Pin F) and -Sense (Pin H) to Common (Pin B)
directly at the mounting kit socket. The maximum voltage
difference between the Bridge Supply, Pin F and the +Sense,
Pin D, is 0.4V.
R EXC (KILOHMS)
Fc
10000
The bridge excitation supply is a very well regulated low
noise output designed to drive either full or half bridge
transducers from 0 to 100mA output current. The output can
be set to a fixed +10V by setting DIP switch SW1 ON. By
setting SW1 OFF the output can be adjusted from +4 to
+10Volts by adjusting the bridge supply adjust trimpot. If a
bridge supply output in the range of +10 to +12 Volts is
desired a user supplied resistor can be installed for R EXC
(SW1 should be set ON), see Figure 3 for proper values of R
EXC to use.
1000
R2 = 16,000/
1000
Bridge Supply
CUTOFF FREQUENCY > 1000 Hz
R1 = 20,000/
100
FIGURE 2. Dip switch settings and equations required to set the
filter cutoff frequency.
Fc
Fc
1000Hz
FREQUENCY (Hz)
Fc
C4 = 0.015 µF
100Hz
-30
1
ALL OFF
1000
-20
-50
Filter Cutoff Frequency Adjustment
SW2
-10
-40
The filter stage is also the input for the output offset voltage
adjustment. The output offset may be adjusted with the on
board trimpot or by driving the output offset input (Pin K) with
a low impedance source or the wiper of a trimpot (see
application section for more information on using this feature).
If an external trimpot is to be used R4 should be removed from
the 160MK, this will disconnect the internal trimpot from any
loading or interaction with the external trim. If the output offset
feature is not desired then connect the External Output Offset
(Pin K) to Common (Pin B) directly at the mounting kit. The
gain from the External Output Offset pin (Pin K) to the filtered
output (Pin N) is approximately 0.7 V/V (i.e. if Pin K is changed
by 1 Volt in a positive direction then Pin N will also change by
0.7 Volts in a positive direction).
Cutoff Frequency
0
160
120
80
40
-1
0
10.0
1000
10.2
10.4
10.6
10.8
11.0
11.2
11.4
11.6
11.8
12.0
BRIDGE SUPPLY (VOLTS)
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FIGURE 3. Graph of R EXC vs. Bridge Supply voltage for excitation
voltage from 10 to 12 VDC
Model 160MK Bridgesensor
parasitics in especially long cable runs to the transducer, it
is suggested that if the transducer load is more than 3 feet
away from the 160MK that a 1 to 10 µF, 35 Volt Tantalum
capacitor be used for C12 and C13 to ensure stability.
SAFE OPERATING AREA CURVE
POWER DISSIPATION (mW)
700
The maximum safe bridge supply current is affected by the
regulator input to output voltage differential and ambient
temperature. The maximum input-output Vs. current is shown
in Figure 4. This graph is for ambient temperatures of 25°C or
below.
DERATING CURVE
125
600
500
400
↑
300
← SAFE OPERATING AREA →
200
↓
100
0
0
10
20
30
40
50
60
70
BRIDGE SUPPLY (mA)
AMBIENT TEMPERATURE (C)
100
FIGURE 5. Graph of maximum internal power dissipation Vs.
ambient temperature.
75
Figure 6 this method is the ultimate for lowering internal
dissipation, the 160MK must supply only the op-amp bias
current. With the TIP31 transistor specified the maximum
bridge supply current is increased to 1 Amp or more (with
proper heat sinking) for any load voltage from 4 to 12 Volts.
This circuit is also useful for running multiple bridges from one
160MK allowing ratiometric measurements to be made with
one 160MK Bridge Supply and Reference driving a common
A/D converter.
50
25
3
4
5
6
7
8
9
10
11
INPUT - OUTPUT VOLTAGE (V)
FIGURE 4. Graph of Bridge Supply maximum output current Vs.
input to output voltage differential
This method is useful in lowering the internal temperature rise
of the hybrid circuit. A lower temperature rise will lower
warmup drift and make the circuit less sensitive to air currents
that might pass along the 160MK. If very high accuracy, and
repeatable measurements are being attempted one of these
methods should be used to lower the power (and hence
temperature rise) in the hybrid circuit.
The ambient temperature derating lowers the maximum safe
output current above 50°C ambient by 2 mA/°C. That is if the
maximum ambient of the 160MK is 65°C the output current
must be lowered by 30 mA ( (65 - 50 ) x 2 = 30 ) over what
Figure 4 shows as a safe maximum.
Figure 5 combines the above information into a single graph
of maximum internal power dissipation Vs. ambient
temperature.
The 6.8 VDC reference (Pin J) can be used as reference for
any other circuitry or an A/D converter, comparator trip level
reference or panel meter reference. The output current of this
pin is limited to 50 µA maximum.
A method of extending the maximum safe Bridge Supply
current is to use an external pass transistor. As shown in
)
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FIGURE 6. Drive very low impedance loads or multiple load cells with an external pass transistor wired on the 160MK’s mating socket.
Model 160MK Bridgesensor
(Pins N or P). Input offset is for amplifier nulling only. Do
not use the input offset for zeroing systems offsets, use
the bridge balance or the output offset adjustments for
system offset correction.
Half Bridge Completion/Bridge Balance
Two 20k ohm thin film resistors are located in the hybrid circuit
from the Bridge Output (Pin 28) to the -Sense (Pin 26). These
resistors form a precision low drift voltage divider for the
Bridge Supply. This circuit can be used as the other half of a
Half Bridge (i.e. one leg or 3 wire) transducer to provide a
common mode voltage to the instrumentation amplifier that
matches the transducer zero offset voltage. This allows
increased gain to be used in the instrumentation amplifier
resulting in higher achievable sensitivity. This voltage divider
is brought out to Pin R on the 160MK and can be directly
connected to either the + or - Input pins.
A companion circuit on the 160MK controls the Bridge Balance.
This circuit is a trimpot connected across the bridge balance
resistors that along with a series resistor (R5) adds or subtracts
a small correction or balancing voltage to the half bridge
completion circuit (about ±5% adjustment range). Normally
Pin S would be connected to Pin R if half bridge completion is
being used. The bridge balance can also be connected
directly to a full bridge transducer to allow nulling and R5 can
be changed on the 160MK to get different zero adjustment
sensitivity.
Using a millivolt calibrator or the transducer output itself,
set the gain so that the proper full scale output voltage is
realized (the mV calibrator or transducer should be set to
simulate full scale output).
3)
If system offsets must be accounted for repeat step 1
again with the inputs disconnected from the source and
connected to ground, then reconnect the inputs and rezero the output with the bridge balance (if used) or the
output offset adjustment.
4)
Steps 1 - 4 above may need to be repeated several times
to achieve the desired accuracy of gain and offset.
160MK Application Examples
Linear NTC Thermistor Oven Controller
A linear NTC (Negative Temperature Coefficient) thermistor
can be interfaced to the 160MK with the following circuit. The
circuit as shown can be used in a precision temperature
controller that can be used to keep various electronic circuits
at a constant temperature for improved performance to study
growth with constant environment variables.
General Calibration Procedures
The 160MK comes from the factory adjusted to the following
specifications:
The transducer used is an ALPHA THERMISTOR INC.
ALN630-1. This thermistor actually consists of 2 thermistors
and a shunt resistor. This arrangement of thermistors produces
a linear output, accurate to within ±0.3°C over a -30 to +50°C
range. The Bridge excitation is set to 4 Volts for minimum self
heating of the thermistor, Rb is chosen to be 22,300 ohms as
per ALPHA’s data sheet.
GAIN .................................... 333 V/V
INPUT OFFSET ................... Adjusted to 0
OUTPUT OFFSET ............... Adjusted to 0
BRIDGE SUPPLY ................ SW1 CLOSED, Bridge Output
at +10 Volts
FILTER ................................. SW2 - SW5 OFF, Filter at 1 kHz
To calibrate the system place the thermistor in an ice bath and
set the input offset to zero output on the DVM, the place the
thermistor in a +50°C hot oil bath and set the gain trimpots for
a +5 Volt reading on the DVM. This procedure may need to be
repeated several times to achieve the desired accuracy. The
DVM will now read directly in °C (be sure to set the DVM’s
decimal place one digit to the right).
BRIDGE BALANCE ............. Pin S at 0 Volts
When adjusting the 160MK to other values the following
methodology should be used,
Ground the inputs and the output offset pins, set the input
offset trimpot to get 0 Volts on the output you will be using
)
FIGURE 7. NTC Thermistor Interfacing Circuit
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1)
2)
Model 160MK Bridgesensor
Digital Scales with Remote Tare Adjustment
the gain trimpots for a full scale reading on the readout. This
procedure may have to be repeated several times to get the
desired accuracy of gain and offset.
The 160MK can be used as the heart of a very effective digital
scale system as shown below. The 160MK is interfaced to a
standard 3mV/V full bridge transducer. With the bridge
excitation set to 10 Volts a gain of 333 V/V is required to get
a 10 Volt full scale output. The output offset pin (K) is used
here to interface to a front panel pot by the display, this pot
allows a tare or dead weight adjustment to be made by the
operator as required to set the system offset to zero reading
for zero weight on the scale (resistor R4 should be removed
from the 160MK to disable any interaction of the internal
output offset adjustment). The system is calibrated by applying
zero weight with the output offset pin and the inputs connected
to ground and adjusting the input offset to get a zero reading,
then remove the inputs from ground and apply a full scale load
to the scale (or simulate one with a mV calibrator) and adjust
Thermocouple Interfacing
The 160MK, while designed for bridge transducer applications
can be used for a variety of other signal conditioning
applications as well. The thermocouple amplifier shown here
can be built for all common thermocouple types by using the
proper resistor values as shown in the table. The amplifier
functions as a differential gain stage with one input sensing
the thermocouple input and the other input sensing the
ambient temperature as derived by the temperature dependant
current of IC1 (AD590). The AD590 functions as an electronic
ice point so that all thermocouple connections can be made at
room temperature.
FIGURE 8. A digital scale application of the 160MK with a remote tare adjustment.
)
FIGURE 9. The 160MK used as a thermocouple amplifier.
Thermocouple
Type
J
K
R
S
T
R1
R TC
RT
348 ohms
267 ohms
75 ohms
75 ohms
275 ohms
154K
154K
348K
348K
154K
51.1 ohms
40.2 ohms
6.04 ohms
6.04 ohms
41.2 ohms
Temperature
R an g e
700°C
1200°C
1400°C
1400°C
400°C
Gain
RG
255
205
624
696
479
316 ohms
392 ohms
130 ohms
115 ohms
169 ohms
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Note: Use stable RN55C 1% metal film resistors for best performance
Model 160MK Bridgesensor
Retrofitting MK165 Applications
Converting MK165 Designs to the 160MK
CALEX has designed the 160MK. to be an attractive upgrade path for existing MK165 applications offering enhanced specifications
and features, while minimizing the changes required to already designed MK165 systems. Below in Table I is a list of the differences
in the pin connections of the 160MK and MK165 and suggested changes.
Table I
Comparison of the 160MK and MK165 mounting kit and pin assignments.
160MK
Function
Mounting Kit
MK165
Function
Suggested Change from
MK165 to 160MK
A
+15 VDC
+15 VDC
No Change Required
B
COMMON
COMMON
No Change Required
C
KEY
COMPARATOR OUT
The 160MK does not implement a level comparator, disconnect this
pin. The key on MK165 applications must be moved to this pin from
pin R in order for the 160MK to seat in the socket properly.
D
+SENSE
+SENSE
No Change Required
E
-15 VDC
-15 VDC
No Change Required
F
BRIDGE SUPPLY
BRIDGE SUPPLY
No Change Required
H
-SENSE
-SENSE
No Change Required
J
6.8V REFERENCE
10.3V REFERENCE
If this pin is used in the MK165 design please note that the 160MK is
a 6.8V reference.
OUT OFFSET
The 160MK output offset is similar in function, but different in voltage
level requirements, please see application information.
K
OUT OFFSET
L
-INPUT
-INPUT
No Change Required
M
+INPUT
+INPUT
No Change Required
N
FILTER OUT
AMPLIFIER OUT
No change required to use filter output feature of 160MK, to use full
bandwidth output connect MK165 pin N to 160MK pin P, but the
inversion in signal may require a reversal in pins L and M.
P
AMPLIFIER OUT
COMPARATOR INPUT
If the MK 165 has pins N and P connected be sure to disconnect, use
pin P on the 160MK for full bandwidth applications, see also note on
pin N above.
R
BRIDGE COMPLETION
KEY
This pin was not used on the MK165
S
BRIDGE BALANCE
COMPARATOR SET
If this pin was connected to external circuitry on the MK165 disconnect
it. On the 160MK this pin is the bridge balance output.
)
160MK Trimpot Adjustments Detail
160MK Trimpot Adjustments
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The 160MK uses the same location on all trimpots and test
points as the MK165 to further reduce the effort required to
upgrade to the 160MK. For most applications no pins need be
changed and the 160MK is a pin for pin enhanced replacement
for the MK165. Be sure to disconnect Pins P and N (if they are
connected on the MK165) as these pins connect the filtered
output an amplifier output together on the 160MK (which will
cause a shorted output condition).
Model 160MK Bridgesensor
Q5) The completed system has more noise
than I thought it would .
Help!
What To Do If You Can’t Get It Working.
A) Try and adjust the cutoff frequency of the filter down to 10
Hz, also be sure that any shielded cables that are used for the
inputs are only grounded at one end. Check that the power
supply is stable and noise free. In general use the lowest
possible filter frequency that you can, this will always minimize
the system noise bandwidth.
Q1) It just won’t work!
A) If the 160MK is wired into a larger system try and isolate the
160MK by disconnecting the input and outputs from the
system. Larger systems are best debugged by disconnecting
all of the functional blocks and starting at the input side then
progressing through the system a functional block at a time.
I also find it helpful to redraw the schematic diagram a block
at a time being sure that I understand where ALL of the current
paths are flowing, usually I can spot the problem when I find
a current path that defies the laws of physics or I spot an open
path. Remember: Divide and Conquer!
Q6) I can’t get the gain and offset adjusted
properly.
A) First be sure that the gain you need is within the internal
trimpots adjustment range of 100 to 500 V/V, if it is not then
you will need to calculate and use a fixed resistor for RG. Be
sure to use the input offset adjustment only when the inputs
are grounded, this will insure that the input offset is in fact only
correcting for the internal offsets of the 160MK. Use the output
offset and bridge balance for any required system offsets.
Q2) OK, it still won’t work!
A) Check the power supply currents with the inputs connected
to Pin B (common). The supply currents should be reasonable
(see the data sheet). Check all the pin connections again, try
and draw the schematic from the wiring that you are looking
at. See if turning any of the trimpots produces any kind of
changes on the output pins (Pins N and P).
Q7) I’ve tried my best but I still need help!
A) By all means feel free to give CALEX applications
engineering a call toll free at 800-542-3355. We spend a lot of
time debugging our own system and can sometimes spot
trouble immediately. Also, we learn a lot about how our
customers use our products so we can make them even more
useful in the future.
Q3) The output appears unstable or is
oscillating.
A) The standard practice of running input lines in shielded
cable should be avoided on the amplifier outputs. These
outputs are low impedance and are not susceptible to noise
pickup, but are susceptible to oscillation due to capacitive
loading as is the case when shielded cable is used. The
filtered output is able to drive up to 1000pF without stability
problems, so be sure to keep the output loading below this
value.
Mechanical Specifications
Q4) The output is at a supply rail or has
severe 60Hz noise on it.
A) The most misunderstood application of instrumentation
amplifiers concerns the return path for the input bias currents.
There must be a path for the input bias currents (small as they
are) to return to system common. If no path is provided then
the inputs will float to an undetermined voltage, usually driving
the amplifier input stage nonlinear and providing a meaningless
and often times noisy appearing output. If the transducer is a
half or full bridge then the path is provided through the bridge
itself, but if the transducer is a thermocouple then the potential
for floating inputs is real. A large resistor ( > 1 Meg) must be
connected from each input to system common to provide for
a ground return.
)
Manufacturing Company, Inc. • Concord, California 94520 • Ph: 925/687-4411 or 800/542-3355 • Fax: 925/687-3333 • www.calex.com • Email: sales@calex.com
9
D-312
If the system doesn’t work properly then examine the input
current paths and be sure that there is a return path for the
input bias currents, if this is not readily apparent then connect
a large resistor to each input to common and see if the system
functions properly.