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LMV551-Q1

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SNOSD24 – FEBRUARY 2017

Product Folder Links: LMV551-Q1

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Copyright © 2017, Texas Instruments Incorporated

Typical Application (continued)

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Allowable Ripple = 1 dB

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Filter Order = Checked and set to 2

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Response Type = Butterworth

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Filter Topology = Sallen-Key

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Component Tolerance - Resistor = E96 1%

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Component Tolerance - Capacitor = E6 20%

After entering these values, FilterPro returns the following recommended values:

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R1 = 44.2 kΩ

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R2 = 38.3 kΩ

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R3 = 2.49 kΩ

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R4 = 22.6 kΩ

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C1 = 10 nF

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C2 = 1.5 nF

The LMV551-Q1 is targeted for low power operation. The above resistor values are assumed for a standard

power application. To save both quiescent and dynamic power, increase the value of the resistors.

The largest consumer of power is the gain setting feedback resistors R3 and R4, as these are DC coupled and

represent a constant DC load to the amplifier. If the output is biased at 2.5 V, then 2.5 V / (22.6 kΩ + 2.49 kΩ) =

99.6 µA is flowing through the feedback network. This is significantly more than the 37uA quiescent current of the

amplifier alone! Increasing the size of the feedback resistors by a decade from 22.6kΩ to 226kΩ, the current in

the feedback network can be reduced down to 9.9µA.

Increasing the resistor values requires a proportional decrease in the values of the capacitors. If a resistor value

is increased 10×, then the corresponding capacitor value must be decreased 10×. However, note that increasing

the resistor values increases the contributed noise, and decreasing the capacitors to small values increases the

sensitivity to stray capacitance.

There is a decision to be made about also scaling the filter components (R1, R2, C1 & C2). R1 and R2 are AC

coupled to the output, so the only DC current flowing through these resistors is the input bias current of the

LMV551-Q1 (typically 20 nA). However, large AC currents can flow through C2 and C1 during large signal

swings. Scaling the filter components also reduces the peak AC signal currents. If the AC signals are expected to

large (several Vpp) and frequent, then scaling the filter values may be beneficial to overall power consumption. If

the expected AC signals are small, it may not be worth the noise tradeoff to scale these values.

Because the LMV551-Q1 has a bipolar input, to maintain DC accuracy, the equivalent resistance seen by each

amplifier input should be equal to cancel the bias current effects.

To maintain DC accuracy through bias current cancelling, the following relationship should be maintained:

(R1 + R2) = (R3 || R4)

(4)

Fortunately, the filter Pro software makes changing and recalculating the values easy. By changing the value of

any of the filter components (R1, R2, C1 & C2) in the schematic tab, the program automatically recalculates and

scale these components. Conversely, changing the gain feedback components (R3 or R4) also causes the other

feedback resistor to scale. However, Filter Pro does NOT maintain the relationship between the feedback and

filter elements as described in Equation 4 above. The feedback resistor values can be 'seeded' and scaled

appropriately, as long as the original feedback resistor ratio is maintained.