Designing programmable logic controller (PLC) analog input modules requires the engineer to balance multiple, often conflicting parameters. Analog input module designs must offer flexibility to support different process signal types and ranges, include multiple inputs for high channel-count systems, enable high-accuracy measurements, and result in a cost-competitive design. This complete, pre-tested front-end design for analog input modules saves development time while demonstrating input flexibility and high-accuracy measurements. Additionally, comparing the 1000-unit cost of this discrete design (without the TPS26611 protection circuit) to a more integrated solution shows that the discrete solution cost is about half. The channel cost is about $0.89 for the discrete solution, which drops further if more analog input channels are added.
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This analog input module front-end design features 8 channels with each channel that can be configured to measure bipolar, universal voltage, and current inputs with a resolution of 16 bits. The module supports a scan rate of 1 ms for all 8 channels. Multiple input voltage ranges are supported with a programmable gain amplifier (PGA). Input ranges supporting up to 20% overrange. Any common-mode voltage (VCM) is effectively rejected. An open-wire detection feature identifies a broken connection to the sensor field transmitter. The design supports the extended industrial operating temperature range of –40°C to 125°C. Table 1-1 summarizes the key parameters of this design.
| Parameter | Value |
|---|---|
| Input channel | 8 |
| Input type | Universal (Current, voltage), bipolar |
| Input mode | Differential or single-ended |
| Differential input signal (maximum) | ±12.288 V |
| Common-mode input signal (maximum) | ±12 V |
| Abs input voltage (each pin) | ±12 V (referred to module ground) |
| Resolution | 16 bit (15+1bit) |
| Sample rate | 8 kSPS |
Voltage input ranges (including 20% overrange) Voltage input ranges (no overrange) Current input ranges | ±10 V ±5 V +10 V +5 V ±2.5 V 0/4...20 mA ±20 mA |
| Input impedance | Voltage mode: > 1 MΩ Current mode: 250 Ω |
| Open-wire detection | Yes |
| 50 or 60 Hz rejection | No |
| Operating temperature | –40°C to 125°C |
All terminals are fully protected against overcurrent up to ±30 mA and overvoltage up to ±33 VDC (permanent). Surges according to IEC61000-4-5 (±1 kV at 42 Ω) are effectively suppressed. Measurements on this reference design board demonstrate 16 bits of effective resolution (or ENOB) and 13–14 noise-free bits (NFB), depending on the selected voltage input range. The accuracy at room temperature is < 0.1%. Table 1-2 summarizes the performance specifications for this design.
| Parameter | Performance |
|---|---|
| Overvoltage protection | ±33 V permanent |
| Overcurrent protection | ±30 mA |
| Surge protection | IEC61000-4-5 (±1 kV/42 Ω) |
| Mis-wiring protection | Yes, voltage and current input mode |
| Effective number of bits (ENOB) | ≥ 16.3 bit |
| Noise-free bits | ≥ 13.7 bit |
| Accuracy error | < ±0.1% at room temp |
| Common-mode voltage rejection | > 80 dB |