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Op amp DC limits, their impact on precision apps

Posted: 13 Jan 2014     Print Version  Bookmark and Share

Keywords:Operational amplifiers  op amps  CMRR  PSRR  data-sheet 

An important conclusion can be made from Equations 12 and 13: for given values of passive resistances and capacitances, the offset voltage is the main contributor to the accumulated output-voltage error.

It is time for an example. Thermal drift of offset voltage (TCVos) and input offset voltage play a very critical role in precision applications where temperature variation is common. To emphasise the significance of TCVOS for an op amp in precision applications, we compare a typical op amp (maximum TCVOS = 5µV/°C and maximum VOS = 50µV) with the MAX9620 (maximum TCVOS = 0.12µV/°C and maximum VOS = 10µV). We can say that:

Maximum VOS(T) = max VOS(+25°C) + maximum TCVOS x (T-25°C) (Eq. 14)

Now we can use the MAX9620 op amp as an example. Assume that in a given application the temperature goes from room temperature (+25°C) to +125°C and that the maximum VOS due to thermal drift is:

Maximum VOS(T) = 10µV + 0.12µV/°C x (100°C) = 22µV .... (Eq. 15)

In contrast, an op amp with a 50µV maximum offset and 5µV/°C maximum TCVOS yields:

Maximum VOS(T) = 50µV + 5µV/°C x (100°C) = 550µV .... (Eq. 16)

These results show the importance of thermal drift for input offset voltage where high accuracy in applications is desired.1

Errors caused by CMRR and PSRR limitations
Finite common-mode rejection ratio (CMRR) in typical op amps degrades precision by introducing an offset voltage at the input. The higher the CMRR of the amplifier, the more insensitive it is to input offset-voltage change over the rated input common-mode voltage. In applications where the input signal is very small, i.e., in the order of mV ranges, high CMRR is absolutely critical.

The CMRR of an amplifier is the ratio of differential gain (ADIFF) to common-mode gain (ACM). CMRR can also be expressed in terms of the change in the input offset voltage with respect to change in the input common-mode voltage (VCM) by 1V. Therefore:

VOUT = ADIFF x [(VIN+ - VIN-) + ACM x VCM/ADIFF] (Eq. 17)

Equation 17 can also be termed as:

VOUT = ADIFF x (VIN+ - VIN-) + ACM x VCM ................... (Eq. 18)

Also:

CMRR = ADIFF/ ACM = delta (VCM)/delta(VOS) (Eq. 19)

Finite power-supply rejection ratio (PSRR) also plays an important role in introducing additional input offset voltage with respect to change in the power-supply voltage. A change in the power-supply voltage (VCC) will alter the operating points of internal transistors which, in turn, affects the input offset voltage. The higher the PSRR, the more insensitive the amplifier will be to the change in input offset voltage when the power-supply voltage is changed.

PSRR = delta (VCC)/delta (VOS) (Eq. 20)

The CMRR and PSRR specs provided in the Electrical Characteristics (EC) table of an amplifier data sheet are specified at a particular input common-mode voltage and power-supply voltage range, respectively, unless otherwise noted. The CMRR spec provided is not the same over the entire power-supply range, and the PSRR spec provided is not the same over the entire input common-mode range.2

Errors caused by input impedance limitations
Finite input impedance (RIN) of an op amp will form a voltage-divider with the source impedance (RS) driving the amplifier and introducing gain error. Consequently, a very high input impedance on the order of tens of 10^9 ohms is required to ensure negligible error.

In the above situation the amount of input signal (VIN) that the amplifier sees from a source depends on the input impedance parameter defined as:

VIN = VSOURCE x [RIN/(RIN+RS)]............................. (Eq. 21)

From Equation 18 if RIN >>> RS, then VIN = VS.

Summary
In conclusion, if DC errors like input offset voltage, input bias currents, and finite input impedance are not addressed, op-amp measurements will simply not be accurate. That performance is not acceptable in high-precision applications where accuracy is paramount. It is also essential that designers understand the significance and limitations of the op-amp performance specs defined in data-sheet EC tables. Following the guidelines presented here, designers can select both the correct op amp and the right passive components with the correct configurations for their applications. Ultimately, using the best op amp for a design will eliminate op-amp errors and ensure the highest accuracy possible.

References
1. The MAX44250 and MAX4238 families of amplifiers also provide maximum input offset voltages in the order of 6µV and 2µV, respectively, which are needed for high-precision mV signal-level amplification in weigh scale and other sensor front-end applications.
2. The MAX44246, MAX44250, and MAX9620 families of amplifiers provide CMRR of 158dB, 140dB and 135dB, respectively, and PSRR of 166dB, 145dB and 135dB respectively. Very high values of CMRR and PSRR are crucial in applications where high-precision DC performance is desired.

About the author
Srudeep Patil is an Applications Engineer working with op amps, comparators, and current-sense amplifiers in Signal Conditioning group at Maxim Integrated since July 2011. He resolves customer issues with technical lab support and works on new product launches by performing IC road tests and writing the data sheets along with the application notes. He has an MSEE with major academic focus on analogue/RF VLSI. Prior to joining Maxim, he worked with NXP Semiconductors as an intern in their analogue team working on ADCs and amplifiers.

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