Building Precision Amplifiers Using Precision Resistor Dividers
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The document introduces a cost-effective method for building precision amplifiers using high-precision digital potentiometers, which outperform mechanical potentiometers in reliability and accuracy. It explains the advantages of digital pots over discrete resistors, highlighting their ability to maintain consistency and temperature tracking since they are manufactured together on the same silicon substrate. Additionally, it details various products from Maxim that offer precise resistor ratio accuracy, thereby enhancing amplifier performance in industrial applications.
Building Precision Amplifiers Using Precision Resistor Dividers
- 1. Building Precision Amplifiers Using Precision Resistor Dividers Source: MAXIM
- 2. Introduction Purpose To introduce a cost effective way to build precision amplifiers to meet the requirements for accuracy demanded by today’s industrial systems using precision resistor dividers. Outline Digital potentiometers replace mechanical potentiometers Why use high precision digital potentiometers MAXIM product portfolio Content 16 pages
- 3. Digital Pots vs. Mechanical Pots Digital potentiometers (Pots) are far more reliable than mechanical potentiometers. Digital pots can easily guarantee 50,000 writing cycles, while the mechanical parts rate only a few thousand. Digital pot resolution can range from 32 steps (5-bit) to 256 steps (8-bit), and more. Digital pots have yet another performance advantage over mechanical potentiometers because of the reduced parasitics and lower susceptibility to noise.
- 4. Attenuator Ratios Buffered Input Buffered Output Standard Resistor Ratios 1:1, 1.1:1, 1.5:1, 2:1, 2.5:1, 3.2:1, 5:1, 7.538:1, 10:1, 25:1
- 5. Negative Feedback Linearizes the Op Amp No Feedback Open Loop Gain 100+ dB 100% Feedback Gain ~1 Voltage Follower Operational Feedback Lots Of Distortion Minimum Distortion + _ + _ R1 R2 IN OUT Distortion? Again It Depends On The Gain And Ratio Of The Resistors
- 6. It’s Still The Ratio Non inverting Inverting The resistor temperature coefficient is dominate and because of feedback the op amp temp coefficient is less important.
- 7. Why Not Just Use High Precision Discrete Resistors? They will not track with temperature The tolerance of two resistors is to an average, not between any two resistors Two discrete resistors are manufactured separately They could be from different manufacturing batches They could be binned
- 8. What’s Binned? It’s a way to deal with manufacturing tolerances with capacitors and resistors. A good process yield is the black standard deviation. But the actual yield could be bifurcated (gray) or off center (green). The result is that most of the time 2% parts will not include 1% parts. 5% parts will not usually have 1% and 2% parts. Except that some months the factory manager doesn’t have enough 5% parts to meet demand with the current months yield. So he puts 1% and 2% in the 5% bin. 2 Percent Bin 1 Percent Bin 10 Percent Bin 5 Percent Bin
- 9. Why Use Maxim High Precision Digital Pots? They will track with temperature They are side by side on the same silicon substrate The tolerance of the two resistors is common to each other Two integrated resistors are manufactured together by the same masks at the same time They are from a die on the same wafer and from the same lot They can’t be separated or binned
- 10. Maxim Precision Digital Pots Resistor Ratio Accuracy Grade A 0.035% MAX Grade B 0.05% MAX Grade C 0.1% MAX Part # Resistance End to End Temperature Coefficient Resistance-Ratio (1:1) Standard Ratios MAX5490 100k ohms 1ppm/ °C Typical 1:1, 2:1, 5:1, 10:1, 25:1 MAX5491 30k ohms 1.5ppm/ °C Typical 1:1, 1.1:1, 1.5:1, 2:1, 2.5:1, 3.2:1, 5:1, 7.538:1, 10:1 MAX5492 10k ohms 2ppm/ °C Typical 1.1:1, 1.5:1, 3.2:1, 7.538:1, 10:1
- 11. Typical Application Circuits MAX5490 MAX5491 MAX5492 MAX5490 MAX5491 MAX5492 Inverting Amplifier Configuration Noninverting Amplifier Configuration
- 12. Precision Voltage Dividers for PGAs Four Gain Settings 1 8 4 2 Voltage Dividers with Precision Matched Resistors Digitally Programmable
- 13. Precision Voltage Dividers for PGAs Offerings Resistor Ratio Accuracy Grade A ±0.025% MAX Grade B ±0.09% MAX Grade C ±0.5% MAX Resistor array makes any op amp a PGA Precision Voltage Dividers and Switches Offset Matching Resistor 8 Pin Package 10 Pin Package Power Supplies ±5 Volts or Single +5V MAX5420 MAX5421 ±15 Volts or Single +15V MAX5430 MAX5431
- 14. Precision Voltage Dividers for Instrumentation Amps MAX5426 Precision Resistor & Switch Network The Classic Three Amplifier Instrumentation Topology Digitally Programmable Differential Gains: 1, 2, 4, 8 Gain Accuracy Grade A 0.025% Grade B 0.09% Grade C 0.5% Dual Supply ±5V to ±15V Adjustable Differential Amplifier Offset
- 15. Conclusion Precision resistor dividers offer a cost effective way to build precision amplifiers to meet the requirements for accuracy demanded by today’s industrial systems. They will track with temperature The tolerance of the two resistors is to each other Two integrated resistors are manufactured together by the same masks at the same time They are from a die on the same wafer and from the same lot They can’t separated or binned
- 16. Additional Resource For ordering MAX549x / MAX542x digital potentiometers, please click the part list or Call our sales hotline For additional inquires contact our technical service hotline For more product information go to http://www.maxim-ic.com/products/digi_pot/ s Newark 1 Farnell Newark 2
Editor's Notes
- #3: Welcome to the training module on Building Precision Amplifiers Using Precision Resistor Dividers. This training module, introduces a cost-effective way to build precision amplifiers to meet the accuracy requirements, demanded by today’s industrial systems.
- #4: Digitally adjustable potentiometers, also known as digital potentiometers, or digipots, are replacing the old-fashioned mechanical versions in all electronic systems and not just in the more well-known audio applications. Digital pots are far more reliable than mechanical pot. Digi-pots can easily guarantee 50,000 writing cycles, while the mechanical parts rate only a few thousand, and sometimes only a few hundred. Digital pot resolution can range from 32 steps or 5-bits, up to 256 steps or 8-bit and sometimes even more. Low resolution is acceptable for LCD-display contrast and other applications in which dynamic range is not critical. Recent high-resolution digital pots are ideal for achieving the typical dynamic range up to 90dB for some audio and hi-fi applications. Beside the improvement in reliability, they use less space and have better electrical performance because of the reduced parasitics and lower susceptibility to noise. Digital pots can replace their mechanical equivalents in virtually every application,
- #5: Precision resistors might seem like an odd product offering from an IC manufacturer. However, precision networks are used as part of building a precise signal path. In a signal path, the performance of the weakest link in the path determines the overall performance of the entire path. Precision resistors provide well matched ratios that allow the high performance amplifiers to achieve their potential. One common usage of a pair of precisely matched resistors is to act as an attenuator to bring the signal range to the common mode range of an op amp or A/D converter input.
- #6: The components inside operational amplifiers are inherently non-linear and cause signal distortion. Normally, negative feedback is used to give the op-amp the capacity to work in it’s linear mode. In the middle of your screen is a simple voltage follower, which takes the op-amp’s output and couples it to the inverting input. As Vin increases, Vout will increase in accordance with the differential gain. However, as Vout increases, that output voltage is fed back to the inverting input, thereby acting to decrease the voltage differential between inputs, which in turn acts to bring the output down. This can really reduce some distortion, but the gain of the amplifier is equal to 1. A precision resistor network allows the feedback loop to linearize the op amp and reduce distortion of the signal. The gain can be adjusted by the value of the two resistors.
- #7: It is the ratio of the resistors that sets the gain for the block. Since the gain is set by the ratio of the resistors and not the overall value of the resistors, matching between the resistors becomes the critical parameter and for systems that have to operate over a temperature range, the temperature coefficient of that matching is also important.
- #8: Discrete resistors have some drawbacks. Predominantly, they do not necessarily track with temperature. They are also not matched to each other but to a nominal value where the resistor is guaranteed as being within a percentage range of that value This is because, the resistors are manufactured and tested in batches. Any two resistors could have been made in a different batch so the temperature coefficient could be different between the two resistors. Matched resistors with 0.1 % tolerance can cost about $5.
- #9: Binning is one additional issue that needs to be noted. When manufacturers are manufacturing resistors or capacitors, they build a batch of the parts for a certain value and then they screen out the different tolerances. This means that a batch of 2% resistors would not have parts with a tolerance between -1 % and + 1 %. Likewise 5% resistors would not include values between -2 % and 2 % and so on. This means that in order to have the best matching, you have to buy the tightest tolerance resistor. Even then, the temperature coefficients of two discrete resistors will not match since they are physically separate components.
- #10: Maxim offers a family of Precision digital potentiometers that are built on a common substrate so they can track with temperature. The ratio can be tightly matched since they sit next to each other on the same substrate and are built in the same batch. They cannot be binned so the tolerance is representative of the process.
- #11: One family of products Maxim offers is the MAX549x. This family offers a pair of resistors with end to end resistances of 10k, 30k, and 100k. The temperature coefficients are very low due to the matching of the resistors and standard ratios are offered for each of the products, allowing the ratio to be selected depending on the application requirement. Three different grades of precision are available and the cost can be balanced with the level of precision required. They are suitable for applications where precision gain setting is needed; such as industrial process control, medical equipment, and instrumentation.
- #12: These diagrams show inverting and non-inverting amplifier configurations using the MAX549x family of precision resistor dividers to accurately set the gain of the amplifier.
- #13: Maxim also has a family of matched resistors combined with on-chip switches so programmable gain amplifiers can be built with an op amp. For example, the MAX5420 uses the precision matched resistors to provide four gain settings: 1, 2, 4, and 8. This product allows the MAX4237 op amp to maintain precision while adjusting the gain for the incoming signal level so that the full range of the A/D converter can be used. Note that the A/D converter is not show..
- #14: The MAX5420 family can support power supplies of +/- 5V or a sing +5V, while the MAX5430 family supports +/- 15 V or a single + 15V. Different grades offer different matching accuracies. A 10 pin version of the part is available and it provides an additional resistor which matches with the matched pair so that bias current errors can be cancelled out.
- #15: Precision resistors can even be used to create programmable gain instrumentation amplifiers. The MAX5426 is an array with two pairs of matched resistors, for creating an instrumentation amplifier with three op amps. As with other products in this family, different accuracies are offered. Gains of 1,2 ,4 and 8 can easily be selected.
- #16: Precision resistor dividers offer a cost effective way to build precision amplifiers and to meet the accuracy requirements demanded by today’s industrial systems. They solve the voltage divider requirements for many applications with very tight tolerances. In this training we’ve introduced the operational amplifier gain control. The ratio of the two resistors determines the amplifier’s output, and not the absolute value of each resistor. The precision resistor divider yields a closer tolerance of ratio matching than 2 discrete resistors of equal absolute tolerance, and better temperature tracking.
- #17: Thank you for taking the time to view this presentation on “ Building Precision Amplifiers Using Precision Resistor Dividers” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the MAXIM site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.