Texas Instruments | Using the XTR115 with the PGA309 to Generate 4mA to 20mA Output (Rev. B) | Application notes | Texas Instruments Using the XTR115 with the PGA309 to Generate 4mA to 20mA Output (Rev. B) Application notes

Texas Instruments Using the XTR115 with the PGA309 to Generate 4mA to 20mA Output (Rev. B) Application notes
Application Report
SBOA107B – June 2005 – Revised April 2006
Using the XTR115 with the PGA309 to Generate 4mA to
20mA Output
Art Kay............................................................................................. High-Performance Linear Products
ABSTRACT
Sensor conditioning systems often require a 4mA to 20mA output, such as in the case
of an instrumentation amplifier translating a pressure sensor output to a usable voltage
level. The combination of the XTR115 and the PGA309 are ideally suited for such
applications. This report discusses component selection to achieve 4mA to 20mA while
setting the desired bandwidth. Grounding considerations are also reviewed.
1
Translating Nonlinear Variations to a Linear Output Voltage
A common requirement of sensor conditioning systems is that they have a 4mA to 20mA output. For
example, instrumentation amplifiers are frequently used to translate a pressure sensor output to a usable
voltage level. In many cases, it is desirable to translate the amplifier voltage output to the
industry-standard 4mA to 20mA. The circuit in Figure 1 shows how the PGA309 voltage output can be
translated to a 4mA to 20mA output. In this circuit, the PGA309 translates nonlinear variations in the
pressure sensor bridge to a linear output voltage (0V to 5V). The PGA309 voltage output is translated into
a current input for the XTR115 current loop transmitter. The XTR115 amplifies the input current and uses
an offset to provide the desired 4mA to 20mA output.
+5V
XTR115
C6
0.1µF
VREG
VSD
SDA
Two−Wire
EEPROM
PGA309
VREF
SCL
RX
RB1
C1
RY
C3
RB2
Input
Amp
RFO
C4
C7
10nF
R2
VIN2
C2
RB3
R1
Voltage
Reference
R3
VOUT_PGA
VEXE
V+
Voltage
Regulator
VSA
C5
I IN
VIN1
RB4
VLOOP
RL
25Ω
RGO
I RET
DGND
AGND
2.475kΩ
25Ω
IOUT
IOUT = 100 IIN
Figure 1. PGA309 and XTR115 Connections for a 4mA to 20mA Output
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Component Selection to Achieve 4mA to 20mA
2
Component Selection to Achieve 4mA to 20mA
Resistors R1 and R2 translate the output voltage of the PGA309 to current [ IIN = VOUT_PGA/(R1 + R2) ]. In
fact, from a DC perspective, resistors R1 and R2 could be replaced with a single resistor. The topology
shown in Figure 1, however, provides an additional first-order filter. Resistor R3 provides an output offset
current for the XTR115; R3 may not be required, however, depending on the PGA309 output range.
Example 1 illustrates a method for setting the output span to 4mA to 20mA when the PGA309 has a fixed
output range. This method uses R3 to set the 4mA offset. Example 2 illustrates how to adjust the PGA309
output range to get the required 4mA to 20mA span without using R3.
2.1
Example 1: PGA309 with Fixed Output Range
Method to select components in order to achieve 4mA to 20mA span using R3 for offset:
Full-scale output of the PGA:
V OUT_FS 4.5V
Zero output of the PGA:
V OUT_Z 0.5V
Current gain of the XTR115:
I
A I OUT
I IN
Resistance required for a 16mA span, given the PGA309 output swing:
V OUT_FSV OUT_Z
VOUT_FSV OUT_Z
R 1R2 I
4.50.5
25.0k
I
20mA
4mA
OUT_FS
OUT_Z
16mA
100
100
100
A
A
I
I
Current with PGA309 output at minimum:
IV
OUT_Z
AI
VOUT_Z
100 0.5
2mA
25.0k
R 1R2
Offset required to set the XTR115 minimum current to 4mA:
I OFFSET 4mAI V
4mA2mA 2mA
OUT_Z
R3 provides the offset current required to set the XTR115 minimum current to 4mA:
V
R 3 I REF 2.5V
125k
2mA
2.2
OFFSET
AI
100 Example 2: PGA309 Output Range without R3 Resistor
Method to select components in order to achieve 4mA to 20mA span by scaling the PGA309 output
(R3 is not used):
Full-scale output of the PGA:
V OUT_FS 4.5V
Zero output of the PGA:
V OUT_Z unknown
Current gain of the XTR115:
I
A I OUT
I IN
2
Using the XTR115 with the PGA309 to Generate 4mA to 20mA Output
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Component Selection to Set Desired Bandwidth
Resistance required to set full-scale output of the XTR115 to 20mA, given a maximum PGA309 output of
4.5V:
V OUT_FS
4.5 22.5k
R 1R2 20mA
20mA
AI
100 PGA309 minimum output required to set the XTR115 output to 4mA:
V OUT_Z R1R 2 4mA 22.5k 4mA 0.9V
100
AI
3
Component Selection to Set Desired Bandwidth
The circuit shown in Figure 1 has three low-pass filters. This section will provide guidelines for setting the
low-pass cutoff frequency for each filter. For each filter in these examples, the cutoff frequency is selected
to be 500Hz. To minimize noise, select the low-pass cutoff to provide the minimum bandwidth allowable
by the given end application.
The first filter consists of C1, C2, and C3. C1 is used to filter the input common-mode noise for the
PGA309. C2 and C3 filter input differential noise for the PGA309. The resistance of the bridge and the
filter capacitors form first-order, low-pass filters. In order to avoid introducing a differential signal by
mismatch errors in C2 and C3, the value of C1 needs to be at least 10 times that of C2.
Note that in some cases, the resistance of the bridge may be low enough to require capacitance values
that are larger than 1µF. This undesirable situation can be avoided by using resistors RX and RY. Example
3 and Example 4 illustrate two methods for selecting these component values.
3.1
Example 3: Component Selection for Input Filter, with RX = RY = 0Ω
Let:
f 3dB 500Hz
If:
R B1 RB2 R B3 R B4 2k
Then the resistance seen by each input of the PGA309 is the parallel combination of two of these
elements.
R BDG RB1 R B2 2k 2k 1k
C1 1
1
0.31F
2RBDGf 3dB
21k500Hz
C 2 C3 C1
0.031F
10
use 0.33F
use 0.033F
SBOA107B – June 2005 – Revised April 2006
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Component Selection to Set Desired Bandwidth
3.2
Example 4: Component Selection for Input Filter, with Non-Zero RX and RY
Let:
f 3dB 500Hz
If:
R B1 RB2 R B3 R B4 100
Then the resistance seen by each input of the PGA309 is the parallel combination of two of these
elements.
R BDG2 RBDGR 501k 1.05k
C1 1
1
0.30F
2RBDG2f 3dB
21.05k500Hz
C 2 C3 3.3
C1
0.030F
10
use 0.33F
use 0.033F
Example 5: Component Selection for PGA309 Filter
For a gain of two and a low-pass cutoff frequency of 500kHz:
1
1
C4 0.018F
2RFOf 3dB
218k500Hz
The PGA309 output can be used as a first-order filter. This filter consists of C4 and RFO. In order to
select components for this filter, we need to know the gain setting that will be used in the PGA309 output
amplifier. The feedback resistance values from Table 1 are used in the formula to compute C4.
Table 1. Resistance Values for PGA309 Output Amplifier Feedback Network
3.4
GAIN
RFO (TYPICAL)
x2
18kΩ
x2.4
21kΩ
x3
24kΩ
x3.6
26kΩ
x4.5
28kΩ
x6
30kΩ
x9
32kΩ
Example 6: Component Selection Filter Between the XTR115 and the PGA309
A final first-order filter consists of C5, R1, and R2. R2 is used to isolate C5 from the XTR115 input
amplifier; a minimum value of R2 = 10kΩ should be used to insure the XTR115 stability. The value of R1
is defined by the XTR115 current scaling requirements. See Example 1 and Example 2 for the current
scaling requirements.
From Example 1:
R 1R2 25k
R 1 25kR2 25k10k 15k
C5 4
1
1
0.021F
2R1f 3dB
215k500Hz
Using the XTR115 with the PGA309 to Generate 4mA to 20mA Output
SBOA107B – June 2005 – Revised April 2006
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Grounding Considerations
4
Grounding Considerations
A common concern regarding this circuit is that the loop supply ground on the XTR115 (or the ground on
the equipment that communicates digitally with the PGA309) can create ground contention. Figure 2
shows a typical system that includes a PC and a PC interface board to facilitate PGA309 programming. It
is critical that the VLOOP supply must be isolated from the PGA309 supply in order to avoid ground
contention, as seen in Figure 2. It is also important that the PGA309, PC Interface Board, and the
computer share a common ground. It is recommended that VPC, the supply connected to the PC interface
board, be a floating supply to eliminate ground loop errors or contention.
DVM
−5.5V
EEPROM
20mA
VREG
PGA309
XTR115
VB
VOUT_PGA
DGND
V+
V−
DVM
IRET
AGND
VLOOP
Floating
Supply
IOUT
RL
250Ω
PRG
−0.5V
V+
V−
VA
PRG
VPC
Floating Supply
VOUT_PGA
PC Interface Board
PC Serial Port Ground
(all GNDs referenced to
this ground)
Serial Port PC
Figure 2. Power Supply and Grounding Connections for PGA309 and XTR115
Doing a nodal analysis on the XTR115 circuit provides insight regarding why the floating supply is required
for VLOOP. The negative end of the VLOOP supply (VB in Figure 2) must be at VB = - RL(IOUT) – (2.475kΩ)IIN
with respect to ground (IRET for the XTR115). For the circuit shown, then, the voltage VB will vary between
–5.5V to –1.1V with respect to ground for a 4mA to 20mA output signal (see DVM1 in Figure 2). It is also
interesting to note that the voltage on the IOUT pin of the XTR115 (VA in Figure 2) varies according to the
following equation:
VA= – (2.475kΩ)IIN.
So, for the circuit shown, VA will vary between –0.1V and –0.5V (see DVM2 in Figure 2).
Furthermore, some care must be taken in the measurement of the output voltage across RL. As seen in
Figure 2, the device or instrument used to measure these voltages needs to be able to measure –5.5V
with respect to PC Serial Port Ground. Specifically, the differential voltage (VB, VA) varies from (–1.1,
–0.1) to (–5.5, –0.5) with respect to PC Serial Port Ground. Therefore, a floating DVM or an
analog-to-digital converter that is capable of measuring signals below ground is required.
SBOA107B – June 2005 – Revised April 2006
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5
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