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Measurement accuracy and stability over time and temperature
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Author: Paul Lee, Marketing Manager, Intersil
ISL28617 simplified block diagram

Figure 1 - ISL28617 simplified block diagram

Industry/Application Area

Target Applications: Industrial, Instrumentation, and Medical Applications

Products Used:

  • Intersil’s ISL28617 (40V Instrumentation Amplifier)
  • VPG’s FRSM Foil Resistors 2512 (120 kΩ, 30 kΩ, 12 kΩ, 1.2 kΩ)

The Challenge:

Design a high performance instrumentation amplifier circuit with high gain accuracy over a wide temperature range (-40 °,Ω C to +125°C). Verification of best solution was accomplished by measuring the performance of the ISL28617 instrumentation amplifier with high-performance thin film resistors vs. Vishay Foil Resistors brand Bulk Metal® foil resistors.

The ISL28617 is a high-performance, differential input, differential output instrumentation amplifier. It can operate over a supply range of 8 V (±4 V) to 40 V (±20 V) and features a differential input voltage range up to ±34 V. The output stage has a rail-to-rail output drive capability optimized for differential ADC driver applications and the outputs can be clamped to ADC supply voltages as low as 3 V (±1.5 V). Its versatility and small package makes it suitable for a variety of general-purpose applications. Fig 1 is a simplified diagram of the ISL28617, showing where the gain of the ISL28617 can be programmed from 0.1 to 10,000 via two external resistors, RIN and RFB.

The gain accuracy is determined by the matching of RIN and RFB. The gain resistors have Kelvin sensing, which removes gain error due to PC board trace resistance. The input and output stages have individual power supply pins, which enable input signals riding on a high common-mode voltage to be level-shifted to a low-voltage device, such as an A/D converter. The rail-to-rail output stage can be powered from the same supplies as the ADC, preserving the ADC maximum input dynamic range and eliminates ADC input overdrive.

The ISL28617 Instrumentation amplifier was developed to accomplish the following:

  • Provide a fully differential, rail-to-rail output for optimally driving ADCs
  • Limit the output swing to prevent output overdrive
  • Allow any gain (0.1)
  • Maximize gain accuracy by removing on-chip component tolerances and external PC board parasitic resistance
  • Maintain CMRR >100dB and remove CMRR sensitivity to gain resistor tolerance
  • Provide a level shift interface from bipolar analog input signal sources to unipolar, and bipolar ADC output terminations
  • Enable user control of amplifier precision level with choice of external resistor tolerance

The problem:

Intersil’s ISL28617 Amplifier

Figure 2 – Intersil’s ISL28617 Amplifier

Intersil’s ISL28617 Amplifier inside a test fixture where R1 and R2 wired connected externally to a PCB including Thin Film or Foil resistors. The heat gun is facing the resistors and changing the ambient temperature up to +125�C.
Figure 3 – Intersil’s ISL28617 Amplifier inside a test fixture where
R1 and R2 are connected externally to a PCB including Thin Film or
foil resistors. The heat gun is facing the resistors and changing
the ambient temperature up to +125°°C
Graph showing the resistance change in ppm from initial value of the resistors (R1 and R2) due to the rise of the ambient change caused by the heat gun

Figure 4 – Graph showing the resistance change in ppm from initial value
of the resistors
(R1 and R2) due to the rise of the ambient change caused
by the heat gun

White line = Z-foil resistor from Vishay Foil Resistors
Yellow line = Thin Film resistor

Since the gain of the instrumentation amplifier is set by the resistor ratio RFB /RIN, resistor temperature drift and instability create gain errors and ultimately limit system accuracy. Even with perfectly matched resistors, board-level environmental conditions may force the resistors to different temperatures, so a low temperature coefficient of resistance (TCR) is desired. For high gains where the resistors are subject to different voltages, resistor voltage coefficients and unequal self-heating can add additional error.

The Solution:

The combination of Intersil's new ISL28617 in-amp with VPG's Bulk Metal Z-Foil resistors provided the ideal high performance solution circuit. These resistors, built on a revolutionary technology, deliver an absolute temperature coefficient of resistance (TCR) of ±0.2 ppm/°C (-55°C to +125°C, +25°C ref.) and ±0.05 ppm/°C (0°C to +60°C, +25°C ref.). By taking advantage of the following features of the overall stability and reliability of Vishay Foil Resistors products, designers can significantly reduce circuit errors and greatly improve overall circuit performance.

  • Temperature coefficient of resistance (TCR)
  • Power coefficient of resistance (PCR)
  • Thermal stabilization
  • Resistance tolerance
  • Load life stability
  • High speed and response time
  • Noise
  • Thermal EMF
  • Electrostatic discharge (ESD)
  • Non-measurable voltage coefficient

The User Explains

To test the performance, the ISL28617 (Fig 2) was tested with both the high-performance thin film resistors and Bulk Metal Foil resistors from VPG. See Fig 3 for the test setup.

As can be seen from Fig 4, the thin film resistor (yellow line) performance is very good at room temperature. But as the ambient temperature changes, the performance degrades very quickly. The foil resistor (white line) performance at room temperature is also very good at room temperature, but as shown it also keeps the same level of performance even with the change in ambient temperature.

Contact Information

Paul Lee

1001 Murphy Ranch Rd

Milpitas, CA 95035

Plee01@intersil.com

+1-408-546-3523

www.intersil.com

Authors:

Paul Lee (Marketing Manager), Intersil In

Don Lafontaine (Application Engineering Manager), Intersil In

Josh Baylor (Design Engineer), Intersil In

Intersil At a Glance

Intersil Corporation is a leader in the design and manufacture of high-performance analog, mixed signal, and power management semiconductors.

Intersil’s products simplify the design process by integrating functions design engineers need to make their products the best in their markets in a more efficient way: By leveraging reliable solutions with the highest level of quality, performance, integration and embedded intelligence, in compact, space-efficient packaging.

Contact Information
  • Paul Lee
  • Marketing Manager
  • Intersil
  • USA, Milpitas,California
  • Email: Plee01@intersil.com
  • Phone: 1-408-546-3523
Customer Statement
"In summary, the stability and reliability offered by VPG resistors maintains the high performance of ISL28617 throughout all environmental conditions. The ultra-low TCR of Bulk Metal® Z-Foil resistors reduces gain error due to thermal gradients and the low PCR provides the best linearity. This is ideal for applications where accurate and stable measurements are necessary regardless of external conditions exerted on the measurement systems."
- Paul Lee
Intersil
Case Study
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