Razumijevanje CMRR kod mjernih pojaalaDamir Mileta

*OutlineDefinitionsdiferencijsko-input pojaalazajedniki naponzajedniki rejection ratio (CMRR)zajedniki rejection (CMR)CMRR za operacijska pojaalaCMRR za Difference pojaalaCMRR za instrumentacijska pojaala

*Diferencijsko pojaaloDiferencijsko pojaalo je sklop koji ima diferencijalni ulaz i pojaava diferencijalni signal dok pri tom potiskuje zajednike signaleTu spadaju operacijska pojaala, instrumentacijska pojaala, i diferencijska pojaalaDiferencijskapojaalainstrumentacijska pojaalaoperacijskapojaala

*Zajedniki naponza diferencijsko pojaala, zajedniki napon se definira kao srednja vrijednost dvaju ulaznih napona. [2]

*Zajedniki napon i diferencijalni naponza diferencijsko pojaala, diferencijalni napon se definira kao razlikadvaju ulaznih napona. [2]

*Pojaanje zajednikog i diferencijalnog naponaIdealno diferencijsko pojaava samo diferencijski napon, ne i zajedniki napon.

*CMRR and CMRCommon-Mode Rejection Ratio se definira kao odnos diferencijskog pojaanja prema zajednikom pojaanju

CMR se definira ovako:

CMR se CMRR esto koriste za u oba oblika

*Real Op-Amp CMRRIn an operacijska pojaala, the diferencijsko pojaanja is known as the open-loop pojaanja. The open-loop pojaanja of an operacijska pojaala is fixed and determined by its design

*Real Op-Amp CMRRHowever, there will be zajedniki pojaanja due to the followingAsymmetry in the circuitMismatched source and drain resistorsTransistor MismatchSignal source resistancesGate-drain capacitanceszaward transconductancesGate leakage currents Output impedance of the tail current sourceChanges with frequency due to tail current sources shunt capacitanceThese issues will manifest themselves through converting zajedniki variations to diferencijsko components at the output and variation of the output zajedniki level. [4]

*Real CMRR ExampleTo understand the effects CMRR can have at the output of device, lets look at an example.OPA376 PDSNotice the Vcm is specified at the top of the pageDeviation from this value will induce an offset errorRemember CMRR is RTI

*Real CMRR ExampleRememberIn reality, CMRR is measured by changing the input zajedniki napon and observing the output change.za an operacijska pojaala, this is usually done with composite pojaalaIt is then referred-to-input by dividing by the pojaanja and can be though of as an offset naponFrom reference [3], in TI datasheets CMRR is defined as follows so that the value is positive

*Real CMRR Exampleza the OPA376, CMRR(min)=76dB. Note this is really CMR!

*CMRR of Difference pojaalaA difference pojaala is made up of diferencijsko pojaala (operacijska pojaala) and resistor network as shown below.The circuit meets our definition of diferencijsko pojaalaThe output is proportional to the difference between the input signals

*DA CMRRThis assumes that the operacijska pojaala is ideal and that the resistors are balanced.Keeping the assumption that the operacijska pojaala is ideal, lets see what happens when an imbalance factor () is introduced.

*DA CMRRAfter some algebra we find that [1]

As expected, an imbalance affects the diferencijsko and zajedniki pojaanjas, which will affect CMRR!As the error->0, Adm->R2/R1 and Acm->0.

*DA CMRRSince we have equations za Acm and Adm, lets look at CMR

If the imbalance is sufficiently small we can neglect its effect on AdmWith that and some algebra we find [1]

*DA CMRRThis equation shows two very important relationships

As the pojaanja of difference pojaala increases (R2/R1), CMR increasesAs the mismatch () increases, CMR decreasesPlease remember that this just shows the effects of the resistor network and assumes an ideal pojaala

*DA CMRRAnother possible source za CMRR degradation is the impedance at the reference pin.So far we have connected this pin to low-impedance ground.

Placing and impedance here will disturb the napon divider we come across during superposition analysis.This will negatively affect CMR

*Real DA CMRR Example (INA149 PDS)

*Why not make our own DA?If DA is simply an operacijska pojaala and 4 resistors, I can save money by making my own, right?

Should be well-matched Should have low temperature drift

*Why not make our own DA?Lets assume an ideal pojaala and just look at resistor mismatches using TINA (only changing R2)Monte Carlo analysisGaussian distribution (6), 100 casesValues are negative due to TINA

Assuming 0% tolerance za R1, R3, and R4 and only 0.1% tolerance za R2 this network can degrade CMRR to 66dB (calculated), 69.16dB (simulated).

*Why not make our own DA?What if all resistors are 0.01% or 0.1%?Worse perzamance than all of our DAs

*Why not make our own DA?0.5%: 52dB (calc), 53.64dB (sim)1.0%: 46dB (calc), 46.85dB (sim)5.0%: 32dB (calc), 33.34dB (sim)

*Why not make our own DA?80dB: Lowest cost of one 0.01%, 10ppm/C resistor (1k pricing)1206 package: $0.45 ($1.80 total cost)0805 package: $0.53 ($2.12 total cost)0603 package: $0.53 ($2.12 total cost)0402 package: $0.50 ($2.00 total cost, 10k pricing!)60dB: Lowest cost 4-pack 0.1%, 25ppm/C resistor (1k pricing)SO-8 package: $0.98 ($0.98 total cost)Footprint size comparison:4 required1 required(need op amp)

0402

0603

0805

1206

SO-8

*Why not make our own DA?Now that we understand how the resistor matching can affect CMRR and the related cost, what about an integrated solution?TI can trim resistors to within 0.01% relative accuracy INA152 CMR(min)=80dBGE=10ppm/C (max)On-chip resistors will drift togetherMSOP-81k price on www.ti.com: $1.20Includes pojaala!Some DAs can give CMR(min)=74dB @ $1.05!Customer will require 2 suppliers (1 za OA, 1 za precision resistors)Op amp included!

MSOP-8

SO-8

*DA pojaanjaWe learned that the pojaanja of difference pojaala is set by R2 and R1.What if we wanted variable pojaanja?We would have to adjust 2 resistors due to the topology.To retain good CMR they would have to be tightly matched, too.This is difficult and expensiveAlternately, you could use an external operacijska pojaala (with very low output impedance so as not to degrade CMR) to drive the reference pin as shown below [4]

*DA pojaanjaBut, R3 should be precision resistor. Its error will be seen as pojaanja error.You also need to purchase an external operacijska pojaala and potentiometer.If you need variable pojaanja, there are better optionsinstrumentacijska pojaala (IAs) usually have an external resistor that can be used to set the pojaanjaProgrammable pojaanja pojaala (PGAs) can be programmed (either with pin settings or digitally) with particular pojaanjaIn summary, difference pojaala are typically manufactured with set pojaanja so as to preserve CMR and since there are alternate (better) solutions za variable pojaanjaSince difference pojaala come with fixed pojaanja, you will only see 1 CMR curve in the datasheet

*Difference pojaala-SummaryPros:Difference pojaala amplify diferencijsko signals and reject zajedniki signalsThe zajedniki rejection is based mainly resistor matchingMaking your own difference pojaala will not yield the same perzamanceDifference pojaala can be used to protect apojaanjast ground disturbancesCons:Externally changing the pojaanja of difference pojaala is not worthwhileThe input impedance is finiteThis means that difference pojaala will load the input signalsIf the input signal sources impedances are not balanced, CMR could be degradedIs there way we can amplify diferencijsko signals, change the pojaanja, retain high CMR, and not load our source?Yes! Buffer the inputsthis creates an instrumentacijska pojaala (IA).

*instrumentacijska pojaalaThere are 2 common types of instrumentacijska pojaala2 op-amp (e.g. INA122)3 op-amp (e.g. INA333)

*instrumentacijska pojaalaNotice both have pojaanja equations so you can vary the pojaanjaNotice the input impedance is that of the non-inverting terminal of non-inverting pojaala Difference AmpHigh-Z NodesHigh-Z NodesVariable pojaanja

*IA CMRRSo, what is the CMRR of an instrumentacijska pojaala?instrumentacijska pojaala reject zajedniki signals (Acm->0)Recall

CMRR is directly related to diferencijsko pojaanja. Since we can change the diferencijsko pojaanja of an IA, we also change the CMRR.

*INA826 CMRR Model Verification

*INA826-Effects of Rg Tolerance on CMRRNow that we see our INA826 model is accurate, lets look at the effects of Rgs tolerance on CMRRSet G=100, 6 resistors, 100 cases.Note that due to the number of cases, no post-processing was perzamedNormally this would be pojaanja/Wavezam. Therezae we have to mentally subtract 20dB from this cluster of wavezams.Notice the pojaanja setting resistor tolerance does not significantly affect the CMR.

*2-OA instrumentacijska pojaalaWhat are the properties of 2-OA instrumentacijska pojaala?ProsLower cost (only 2 op-amps), less trimmingHigh impedance inputCan be placed in smaller packageConsCompare signal path to Vo za Vin+ and Vin-Vin+ has shorter path than V-This delay does not allow the zajedniki components to cancel each other as well as frequency increasesTherezae CMR degradation occurs earlier in frequency than the 3-OA designsSince we can change the diferencijsko pojaanja, the CMR also changes.

*Real IA CMR Competitive Analysis

*SummaryA diferencijsko pojaala amplifies diferencijsko signals, not zajedniki signalsExamples include operacijska pojaala, difference pojaala, and instrumentacijska pojaalaCMRR is defined as the ratio of diferencijsko pojaanja to zajedniki pojaanjaAll diferencijsko pojaala have an ideal zajedniki pojaanja of 0To determine if circuits CMRR is going to change with pojaanja, you must look at the diferencijsko pojaanja. Remember an op-amps diferencijsko pojaanja is fixed.If you can change the diferencijsko pojaanja of the device/circuit, the CMRR will also change

****A diferencijsko input pojaala is the same as an operacijska pojaala.

A difference pojaala is an operacijska pojaala with surrounding resistors.*********************************