![[Tech-On!]Tech News-Straight from Asia](/english/img/06/logo1.gif){kind=logo}
Op-Amp Offers 0.25mV Offset Voltage for Digital Age
| Geographic Eligibility (Edition) | Medium (Click where applicable) |
|---|---|
| Hong Kong, Malaysia, the Philippines, Singapore and Thailand (English) | Magazine + e-newsletter E-newsletter only |
| India (English) | Digital magazine + e-newsletter E-newsletter only |
| Taiwan (Complex Chinese) |
Magazine + e-newsletter E-newsletter only |
| South Korea(Korean) | Magazine + e-newsletter E-newsletter only |
| Other countries/regions | E-newsletter only *Readers from other countries/regions who want to receive Nikkei Electronics Asia printed magazine may consider paid subscription. Please contact NE Asia Circulation Department for details. |
In the embedded control designs of today, it is often necessary to analyze parameters occurring in the analog world. Although all the control decisions are made in the digital realm, those decisions are often based on occurrence in the analog realm. Converting an analog signal to a digital value has been greatly simplified with a number of off-the-shelf, stand-alone analog-to-digital converters (ADC) and microcontrollers with built-in ADC devices. Unfortunately the analog signal presented to the system is often not in a form that accurately converts to a digital value.
Many of the sensors needed to measure the parameters like temperature and pressure produced a signal not suitable for conversion. The signal is either too weak to be converted or contains high frequency noise that will alias into the digital value during conversion. For these reasons operational amplifiers are often used to gain up the signal and provide an active filter to remove high frequency noise.
Many operational amplifiers have bias current and offset voltage large enough to cause significant error when used in these applications if special care is not taken in design. Even then, performance can be limited by offset voltage and bias current, unless added components are used. These issues can be avoided by the use of new families of operational amplifiers with low offset voltage designed for digital systems and priced below the high premium for such precision operational amplifiers of the past.
A typical application with a gain of 100 can lose a lot of the output voltage range due to an error voltage from the offset voltage. In this case a 5V supply would mean 20% or more of the range could not be used. This would become worse at lower voltages where 3V would lose 33% or more of the output range.
With an operational amplifier that has less than 0.25mV of offset voltage, this results in only 25mV of error voltage at the output or less than 0.5% of the output range with a 5V supply (see Fig).
In the inverting configuration (not shown) voltage dropped across the input resistor due to bias current can cause error in the output. Caution must be taken by the designer not to use too large a value for that resistor. The sensor itself may not be able to supply the large value of bias current many amplifiers require. These situations can be completely avoided by using an operational amplifier with very low bias current.
Compensation Values
The recently introduced MCP606 (single), MCP607 (dual), MCP608 (single with chip select) and MCP609 (quad) operational amplifiers from Microchip Technology are a family of such amplifiers. These amplifiers were designed to operate on a single supply of 2.5 - 5.5V, a range of voltage typically available in microcontroller and DSP systems. Further system supply demands were accounted for by only 20 microamperes of quiescent current per amplifier. These devices are suitable for the battery requirements of the systems of today.
Given the low bias current (IB < 1 pA), low offset voltage (Vos < 250mV) and the fact that these devices are unity gain stable, many of the tricky analog design concerns have been eliminated. This allows the digital designer to get to market quicker.
Ironically, it is digital control that makes this family of operational amplifiers possible. Non-volatile memory is used to store offsetting values at final test. These values are used in the amplifier to compensate for the offset voltage. Because the values are determined at final test, this also means that the effects that assembly can have on offset voltage are compensated.
Trimming techniques of the past required expensive laser equipment and could only be done at the wafer level. Once the part was packaged no changes could be made.
The non-volatile memory technique enables compensation on the finished product allowing for greater accuracy without expensive laser trim equipment. Digital control circuitry is being used to bring an analog product to market that is well behaved for the digital designer.
More and more analog products are becoming available with the concerns of the digital system designer in mind. Recognizing the world will remain analog, despite the great strides of digital control, manufacturers are offering ways to bridge the gap and meet the demands of the digital age.
by Art Eck
product marketing manager, Microchip Technology Inc, USA
(September 2000 Issue, Nikkei Electronics Asia)
Many of the sensors needed to measure the parameters like temperature and pressure produced a signal not suitable for conversion. The signal is either too weak to be converted or contains high frequency noise that will alias into the digital value during conversion. For these reasons operational amplifiers are often used to gain up the signal and provide an active filter to remove high frequency noise.
Many operational amplifiers have bias current and offset voltage large enough to cause significant error when used in these applications if special care is not taken in design. Even then, performance can be limited by offset voltage and bias current, unless added components are used. These issues can be avoided by the use of new families of operational amplifiers with low offset voltage designed for digital systems and priced below the high premium for such precision operational amplifiers of the past.
A typical application with a gain of 100 can lose a lot of the output voltage range due to an error voltage from the offset voltage. In this case a 5V supply would mean 20% or more of the range could not be used. This would become worse at lower voltages where 3V would lose 33% or more of the output range.
With an operational amplifier that has less than 0.25mV of offset voltage, this results in only 25mV of error voltage at the output or less than 0.5% of the output range with a 5V supply (see Fig).
In the inverting configuration (not shown) voltage dropped across the input resistor due to bias current can cause error in the output. Caution must be taken by the designer not to use too large a value for that resistor. The sensor itself may not be able to supply the large value of bias current many amplifiers require. These situations can be completely avoided by using an operational amplifier with very low bias current.
Compensation Values
The recently introduced MCP606 (single), MCP607 (dual), MCP608 (single with chip select) and MCP609 (quad) operational amplifiers from Microchip Technology are a family of such amplifiers. These amplifiers were designed to operate on a single supply of 2.5 - 5.5V, a range of voltage typically available in microcontroller and DSP systems. Further system supply demands were accounted for by only 20 microamperes of quiescent current per amplifier. These devices are suitable for the battery requirements of the systems of today.
Given the low bias current (IB < 1 pA), low offset voltage (Vos < 250mV) and the fact that these devices are unity gain stable, many of the tricky analog design concerns have been eliminated. This allows the digital designer to get to market quicker.
Ironically, it is digital control that makes this family of operational amplifiers possible. Non-volatile memory is used to store offsetting values at final test. These values are used in the amplifier to compensate for the offset voltage. Because the values are determined at final test, this also means that the effects that assembly can have on offset voltage are compensated.
Trimming techniques of the past required expensive laser equipment and could only be done at the wafer level. Once the part was packaged no changes could be made.
The non-volatile memory technique enables compensation on the finished product allowing for greater accuracy without expensive laser trim equipment. Digital control circuitry is being used to bring an analog product to market that is well behaved for the digital designer.
More and more analog products are becoming available with the concerns of the digital system designer in mind. Recognizing the world will remain analog, despite the great strides of digital control, manufacturers are offering ways to bridge the gap and meet the demands of the digital age.
by Art Eck
product marketing manager, Microchip Technology Inc, USA
(September 2000 Issue, Nikkei Electronics Asia)
