Operational Amplifier Precision Design Example
This section goes over the thermal design of a thermocouple Printed Circuit Board (PCB) available from Microchip. This PCB has the following descriptors:
- MCP6V01 Thermocouple Auto-Zeroed Reference Design
- 104-00169-R2
- MCP6V01RD-TCPL
The application of this PCB can be found in this user’s guide: “MCP6V01 Thermocouple Auto-Zeroed Reference Design”
Circuit Description
Figure 1 shows the general functionality of this design (the schematic is shown on PCB Parasitic Resistance).
Figure 1
The (type K) thermocouple senses temperature at its hot junction (TTC) and produces a voltage at the cold junction (at temperature TCJ). The conversion constant for type K thermocouples is roughly 40 μV/°C. This voltage (VP – VM) is input to the Difference Amplifier (MCP6V01).
The MCP9800 senses temperature at the Type K Thermocouple’s cold junction (TCJ). The result is sent to the PICmicro™ microcontroller via an I2C bus. The firmware corrects the measured temperature for TCJ.
The difference amplifier uses the MCP6V01 auto-zeroed op amp to amplify the thermocouple’s output voltage. The VREF input shifts the output voltage down so that the temperature range includes -100 °C. The VSHIFT input shifts the output voltage, using a digital POT internal to the microcontroller (CVREF), so that the temperature range is segmented into 16 smaller ranges; this gives a greater range (-100 °C to +1000 °C) with reasonable accuracy.
The MCP1541 provides an absolute reference voltage because the thermocouple’s voltage depends only on temperature (not on VDD). It sets the nominal VOUT and serves as the reference for the Analog-to-Digital Converter (ADC) internal to the microcontroller.
The 2nd Order, Low-pass Filter reduces noise and aliasing at the ADC input. A double R-C filter was chosen to minimize Direct Current (DC) errors and complexity.
CVREF is a digital POT with low accuracy and highly variable output resistance. The buffer (×1 amplifier) eliminates the output impedance problem, producing the voltage VSHIFT. Since CVREF has 16 levels, we can shift VOUT1 by 16 different amounts, creating 16 smaller ranges; this adds four bits resolution to the measured results (the most significant bits). The 10 bits produced by the ADC are the least significant bits; they describe the measured values within one of the 16 different smaller ranges. VSHIFT is brought back into the PICmicro microcontroller so that it can be sampled by the ADC. This gives VSHIFT values the same accuracy as the ADC (10 bits), which is significantly better than CVREF’s accuracy. The measured value of VOUT2 is adjusted by this measured VSHIFT value.
The overall accuracy of this mixed-signal solution is set by the 10-bit ADC. The resolution is 14 bits, but the accuracy cannot be better than the ADC since it calibrates the measurements.
PCB Layout
In the figures in this section, the red numbers (inside the circles) point to key design choices, which are described by a list after each figure.
Figure 2 shows the top silkscreen layer of the PCB designed for the MCP6V01 Thermocouple Auto-Zeroed Reference Design.
Figure 2
1. The Difference Amplifier is as close to the sensor as possible and is on the opposite PCB surface from the PICmicro microcontroller. This minimizes electrical and thermal crosstalk between the two active devices.
2. Small resistors (0805 SMD) reduce the thermoelectric voltages, for a given temperature gradient.
3. The resistors that are a part of the Difference Amplifier play a critical role in this design’s accuracy.
- R6 and R7 are at the input from the thermocouple, and give a gain of 1,000 V/V to VOUT1.They are arranged so that their thermoelectric voltages cancel.
- R9 and R10 are at the input from the range selection circuitry (VSHIFT), and give a gain of 17.9 V/V to VOUT1. Changing their location and orientation on the PCB might improve the performance.
- R8 and R11 convert the inputs to the output voltage (VOUT1). Changing their location and orientation may not improve the performance enough to be worth the trouble. Figure 3 shows the top metal layer of the PCB. The sensitive analog and sensor circuitry is connected to this layer.
Figure 3
4. Metal fill, connected to the ground plane, minimizes thermal gradients at the cold junction connector.
5. The MCP9800 Temperature Sensor (cold junction compensation) is centered at the cold junction connector to give the most accurate reading possible.
6. Sensor traces are separated from power (top layer) and digital (bottom layer) traces to reduce crosstalk.
7. The MCP9800’s power traces are kept short, straight and above ground plane for minimal crosstalk.
Figure 4 shows the power plane. It minimizes noise conducted through the power supplies and isolates the analog and digital signals.
Figure 4
8 The power plane on the left helps keep the temperature relatively constant near the auto-zeroed op amp. It also provides isolation from the microcontroller’s electrical and thermal outputs.
9. The power plane on the right helps keep the temperature relatively constant near the thermocouple’s cold junction and MCP9800 cold junction temperature sensor.
10. The FR4 gap provides attenuation to heat flow (a relatively high-temperature drop) between the active components on the left (MCP6V01 and PIC18F2550) and the sensors on the right (thermocouple and MCP9800).
Figure 5 shows the ground plane. It also minimizes noise conducted through the power supplies and isolates the analog and digital signals.
Figure 5
11. Same function as #8.
12. Same function as #9.
13. Same function as #10.
14. This ground plane extension provides better isolation between digital signals and the MCP9800’s power supply. It also helps protect the thermocouple signal lines. However, it increases the thermal conduction between the left and right sides of the PCB.
Figure 6 shows the bottom silk layer.
Figure 6
15. The USB connector and its components are isolated from the rest of the circuit.
16. The crystal (XTAL) oscillator is as far from everything else as possible, except the clock input pins of the microcontroller.
17. The microcontroller produces both thermal and electrical crosstalk, so it is isolated from the analog components.
Figure 7 shows the bottom metal layer of the PCB. The digital circuitry is connected to this layer.
Figure 7