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Part: AN203
Category:
Description: Silicon-gate Switching Functions Optimize Data Acquisition Front End
Company: Vishay Intertechnology
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AN203
Vishay Siliconix
Silicon-Gate Switching Functions Optimize Data Acquisition Front Ends
The trend in data acquisition is moving toward ever-increasing accuracy. Twelve-bit resolution is now the norm, and sixteen bits are not uncommon. Along with this precision, throughput is also very important. When monitoring several hundred channels, sample rates in the hundreds of kilohertz are not only desirable but, in many cases, mandatory. Analog switches and analog multiplexers find extensive use at the heart of most data acquisition and process control systems. This application note provides useful information about the new high-performance DG400 family of devices. It also reviews many design considerations that will enable you to get the best performance in your data acquisition designs.
operations. This produces high gate-drain and gate-source capacitances. The silicon-gate process, on the other hand, is self-aligning in that it uses the silicon gate itself as a mask for source and drain diffusions. This produces minimal overlap, resulting in much smaller parasitic capacitances. Because the silicon-gate process is more tightly controlled than the older metal-gate technologies, individual devices can be spaced closer together, resulting in smaller die that achieve equivalent performance.
ESD Tolerance Electrostatic discharge (ESD) has caused many CMOS device failures, both during manufacture and during handling or PC board assembly. Historically, CMOS devices have shown an electrostatic discharge sensitivity (ESDS) in the "500-V range, which was insufficient in many cases. However, the DG400 family incorporates specially designed ESD protection. These devices have been evaluated using the electrostatic discharge sensitivity (ESDS) test circuit of MIL-STD-883, Method 3015 (100-pF capacitor discharged through a 1.5-kW resistor). The DG4XX series has a typical overall tolerance of 1000 V. However, ESD tests on the source/drain--with the power supply pins bypassed or shortened--show that the DG400 through DG405 have tolerances of more than "2000 V, whereas the DG408 through DG419 withstand > "4000 V.
Silicon-Gate Technology Vishay Siliconix's advanced high-voltage silicon-gate CMOS processing brings many benefits to the DG400 family of analog switches and multiplexers: fast switching speed, low power consumption, low charge injection, low leakage, and TTL compatibility. In addition, this family works with reduced or single power supplies. The metal-gate process (Figure 1) requires that the gate overlap with the drain and source areas to assure reliable operation even when misalignments occur during masking
S
G
D
S
G
D
Oxide
p
p
p
n a) Metal-Gate MOSFET
b) Silicon-Gate "Self-Aligned' MOSFET
FIGURE 1. Comparison of Metal and Silicon-Gate Structures
Document Number: 70601 06-Aug-99
ΝΝΝΝΝ ΝΝΝΝΝ
Poly-Si Oxide n
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AN203
Vishay Siliconix
Typical Data Acquisition System Operation of the analog front end is governed by means of a digital controller which, in turn, interfaces to a host computer or microprocessor. Digital event inputs or interrupts go directly to the controller, and its digital outputs provide the feedback necessary to perform automatic process control. In addition to the analog multiplexer, analog switches are found in the PGA, S/H, and ADC circuits. The following paragraphs will review many design considerations as we proceed to the design and evaluation of an experimental data acquisition system front end.
Figure 2 shows the block diagram of a typical data acquisition system. Analog inputs are converted to a digital format that allows a computer to gather, monitor, display, and analyze the collected data. If the system has digital output capabilities, the computer can be used to accurately control your process so it will run at maximum efficiency. For example, it can react to the input data to maintain a constant temperature, to control flow rates in accordance to a predetermined schedule, etc.
This system accepts analog voltage inputs that can come from temperature sensors, pressure transducers, flow meters, or from optional signal conditioners or remote current-mode transmitters. The signal conditioning stages can perform preamplification, scaling, and multiplexing, and can also provide galvanic isolation or overvoltage protection. The analog multiplexer is basically a monolithic array of analog switches with on-chip address decode logic. The multiplexer is a cost-effective solution that shares the more expensive sample-and-hold (S/H) and analog-to-digital converter (ADC) functions among several inputs. The programmable-gain amplifier's (PGA) purpose is to amplify low-level signals to increase measurement resolution and accuracy. The S/H circuit quickly captures a sample of the analog input signal and holds its instantaneous value for a time that is long enough to allow for the ADC acquisition time to be completed.
Designing an Experimental Temperature Monitoring System
The purpose of our experimental circuit is to evaluate some of the errors introduced while monitoring temperature via three popular types of transducers: a two-terminal integrated circuit, a resistive temperature detector (RTD), and a type J thermocouple. In addition, we want to sample ground and a +5-V reference voltage. These two quantities may be used for calibration and error correction purposes. Signals from the sensors will arrive at the PC board via twisted pair wires to help eliminate any common-mode noise. No cold-junction compensation of the thermocouple will be attempted since compensation circuits are readily available. The schematic diagram is shown in Figure 3.
From Sensors, Transducers, etc.
A
LPF
Analog Inputs
Analog Multiplexer
PGA
Sample and Hold
12-Bit A/D Converter
Digital Inputs Digital Outputs Digital Controller Data Bus
FIGURE 2. A Typical Data Acquisition System
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Document Number: 70601 06-Aug-99
AN203
Vishay Siliconix
Sensors Mux PGA S/H
15 V 100 W 0.01 Sensor 1 AD590 +5 V 4 5 510 pF Sensor 2 RTD (100 W) J107 +15 V 15 mA IN5819 D1 +5 V Sensor 3 Type J Thermocouple +15 V 9 10 kW MUX 7 12 S4 S5 DG408 S8 14 S1 S2 3
+15 V 100 W 0.01 13 20 kW VA 12 1 5 8 3 + LF356 2 4 15 V +5 V 5 +15V 4 8 2 Gain Select H=2 L = 1000 6 DG419 3 7 100 W 15 V 0.01 A = 1000 100 W A=2 10 kW 100 pF 10 kW 7 6 +15 V VB 16 30 W 4 S/H 15 13 99.9 kW 100 W
1/ DG405 2
+5 V
+15 V R1
100 W 0.01 11
100 W 0.01
1500 pF +15 V 20 kW
7 1 0.001 3 300 W 3 + 2 1 LF356 4 15 V 0.01 1500 pF Polystyrene 56 VOUT
14
DGND
1
15 V
FIGURE 3. Temperature Monitoring Circuit
a) On-Resistance 320 r DS(on Drain-Source On-Resistance ( W ) ) 200
b) Drain On-State Leakage Current
DG408 240 DG508A I D(on)(pA) 0
160
200
DG508A 80 DG408 0 15 600 15 400
10
5
0
5
10
15
10
5
0
5
10
15
VS Source Voltage (V)
VS Source Voltage (V)
FIGURE 4. DG408 Typical Characteristics
The Multiplexer
Ve max [ 40 W
100 pA [ 4 nV
The DG408 is an 8-channel single-ended multiplexer with an on-chip logic reference that maintains TTL compatibility over a wide range of power supply voltages. Its low on-resistance and low leakage (see Figure 4) minimize static errors. The worst-case error due to leakage is given by
Document Number: 70601 06-Aug-99
Figure 5 shows a thermocouple representation of one switch in the multiplexer. If connections J1S and J1D are at the same temperature, their thermoelectric EMFs will cancel out. If a temperature gradient exists between side "S" and side "D," the voltages will not exactly cancel, and a net error voltage will result. Therefore, the multiplexer should be mounted in a thermally stable environment--that is, soldered to the PC
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AN203
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board and away from hot components and air drafts. The DG408, thanks to its low power dissipation (1 mW), develops less than "1 mV in still air at room temperature. When heated to 85_C with a thermal probe, the error becomes as large as 100 mV. On-resistance (rDS(on)) matching is necessary when the rDS(on) of the multiplexer is significant when compared to the rest of the circuit, such as when using differential multiplexing or when the transducer is connected in a resistive bridge configuration. Having smaller rDS(on), the DG408 offers also an "rDS(on) matching" specification which is three times better than the DG508 in terms of the magnitude of resistance.
PCB Trace S J 1S IC Lead
The circuit in Figure 6b uses four resistors to achieve the same two gain values as the previous example. In comparing the two circuits, it would seem that the first circuit is the better of the two since it requires fewer resistors. Upon closer examination, however, we find that there's more to consider. In Figure 6a, the analog switch is in series with the feedback resistor. This means that the rDS(on) of the switch is part of the feedback ratio. Instead of the gain being AV = Rf/R1, it becomes
AV = (Rf + rDS(on))/R1
Bond Wire
While the lower on-resistance and lower DrDS(ON) of the DG400 family of switches do offer advantages over older metal-gate switches, in this situation, it would be better still if we could eliminate the effect of the switch rDS(ON) altogether. This is where the circuit shown in Figure 6b has an advantage. As shown in Figure 6b, the gain in each stage is determined by the two resistors in that branch. For the gain of 10, for example, the gain is
AV = (Rf1 = Rg1)/Rg1 = (18 + 2)/2 = 10
Chip Metalization
p-Channel Silicon
n-Channel Silicon
Chip Metalization
Bond Wire
The rDS(on) of the switch has no affect at all on the gain! However, the switch leakage current may affect the circuit accuracy. Here, again, the DG400 family of switches have an advantage because their leakage current is much lower than the metal-gate switches. By choosing precision resistors or resistor networks, it is possible to have gains accurate to the 12-bit level. The output voltages of the AD590 and RTD in Figure 3 are greater than 1 V in the temperature range used. The thermocouple, however, had an output voltage that varied from a few microvolts to several millivolts. Therefore, two different gains are required from the PGA: a relatively low gain of two for the AD590 and RTD, and a gain of 1000 for the thermocouple. This allows the voltages to the sample-and-hold to be large enough that any errors contributed by the circuit would have minimal effect. The DG419 used to select the gain is a very low-power, high-speed switch. An additional benefit of this particular switch is its compact 8-pin package The 30-W resistor serves two purposes. It limits the current through the 1/2 DG405 to less than its 100-mA (maximum pulsed) rating and also helps to decouple the PGA output from the capacitive load, preventing oscillations. For best results, in applications where high gains are required due to very low-level transducer outputs, as with strain gages and thermocouples, the signal path to the PGA should be differential.
Document Number: 70601 06-Aug-99
IC Lead J 1D PCB Trace D
FIGURE 5. Thermocouple Representation of a CMOS Multiplexer Switch
The Programmable Gain Amplifier There are several types of gain-ranging circuits. Although it would be impossible to cover them all here, Figure 6 shows two of the most common types. Each of these types has advantages and disadvantages. Figure 6a shows a circuit which uses three resistors and two switches. By closing one switch at a time, two different gains can be selected. By closing more than one switch at a time, three gain combinations are possible.
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AN203
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VIN Rf1 Rf2 VIN R1 + a) VOUT S2 Rg1 b) Rg2 S1 S2 S1 + VOUT Rf1 Rf2
FIGURE 6. DG408 Typical Characteristics
5V SAMPLE/HOLD Command 5V
VOUT
500 mV/div
1 V/div
0V 2 ms/div
5 V 2 ms/div
FIGURE 7. Acquisition Time Depends on Amplifier Slew Rate
The Sample-and-Hold Circuit The sample-and-hold (S/H) circuit uses a 1/2 DG405, a fast (tON < 250 ns) switch. In this circuit, the two switch sections are at similar potentials in the sample mode, so when they open, they create similar charge injections which tend to cancel each Evaluation Results
other, therefore, helping to minimize the step error. R1 can be trimmed to obtain the best possible charge injection cancellation. During the hold mode, the dual switch arrangement also helps to reduce the droop rate. Figure 7 is a scope plot that illustrates the S/H action. Note that acquisition time is a function of the output amplifier's slew rate and settling time. produce a larger DV and offer a resolution equivalent to 0.01_C/bit. Figure 8 shows the waveforms obtained when switching back and forth between channel S2 (AD590) and S4 (RTD). Note that the PGA output takes longer to settle when the AD590 is selected. On the other hand, the lower output impedance of the RTD sensor makes the PGA output settle about three times faster. From these waveforms, we can estimate the throughput of the system. Allowing 20 ms for settling times and assuming a 12-bit A/D converter with a 15-ms conversion time,
Throughput rate = 1/35 ms = Y28 kHz
Figure 9 shows the transfer characteristics obtained for the three different temperature sensors used. Curve (c) is produced by the "mV output" of a digital thermometer, using the same thermocouple that produced curve (d). Notice the effect that the cold-junction compensation has on the curve (i.e., it causes a 0-V output at 0_C).
As far as resolution is concerned, all three sensors showed satisfactory results for the 0 to 100_C range evaluated. The thermocouple output gave a resolution equivalent to 0.05_C/bit in a 12-bit system. The RTD and AD590 outputs, as configured, only gave the equivalent of 0.5_C/bit and 1_C/bit resolution, respectively. However, depending on the temperature range of interest, this circuit can be modified to
Document Number: 70601 06-Aug-99
Precision will depend on the method used to read the transfer characteristics. Factors such as ADC accuracy, transducer accuracy, noise corruption, leakage throughout the signal path, and amplifier offsets must be considered.
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