OPA835IDBVT datasheet(35/53 Pages) NSC

35 / 53 page

OPA 835

10 W

100 pF

From AP

+2.5 V

-2.5 V

To AP

OUT

-40

-30

-20

-10

10k

100k

f - Frequency - Hz

Sallen-Key

MFB

V = 5 V,

= 100 mV

OUT

OPA835

OPA2835

www.ti.com

SLOS713E – JANUARY 2011 – REVISED JULY 2013

Figure 74. MFB and Sallen-Key 2nd Order Low-Pass Butterworth Filter Response

MFB and Sallen-Key filter circuits offer similar performance. The main difference is the MFB is an inverting

amplifier in the pass band and the Sallen-Key is non-inverting. The primary pro for each is the Sallen-Key in unity

gain has no resistor gain error term, and thus no sensitivity to gain error, while the MFB has inherently better

attenuation properties beyond the bandwidth of the op amp.

Audio Frequency Performance

The OPA835 and OPA2835 provide excellent audio performance with very low quiescent power. To show

performance in the audio band, a 2700 series Audio Analyzer from Audio Precision was used to test THD+N and

FFT at 1 VRMS output voltage. Figure 75 is the test circuit used. Note the 100 pF capacitor to ground on the input

helped to decouple noise pick up in the lab and improved noise performance.

Figure 76 shows the THD+N performance with 100 k

Ω and 300 Ω loads, and with no weighting and A-weighting.

With no weighting the THD+N performance is dominated by the noise for both loads. A-weighting provides

filtering that improves the noise so a larger difference can be seen between the loads due to more distortion with

RL = 300 Ω.

Figure 77 and Figure 78 show FFT output with a 1 kHz tone and 100k

Ω and 300Ω loads. To show relative

performance of the device versus the test set, one channel has the OPA835 in line between generator output

and analyzer input and the other channel is in “Gen Mon” loopback mode, which internally connects the signal

generator to the analyzer input. With 100 k

Ω load, Figure 77, the curves are basically indistinguishable from each

other except for noise, which means the OPA835 cannot be directly measured. With 300

Ω load, Figure 78, the

main difference between the curves is OPA835 shows higher even order harmonics, but odd order is masked by

the test set performance.

Figure 75. OPA835 AP Analyzer Test Circuit

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OPA835IDBVT Datasheet(PDF) 35 Page - National Semiconductor (TI)

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