Datasheet

OPA835
10 W
100 pF
V
From AP
IN
+2.5 V
-2.5 V
V
To AP
OUT
-40
-30
-20
-10
0
1k 10k 100k
1M
f - Frequency - Hz
Sallen-Key
MFB
Gain Magnitude - dB
V = 5 V,
V = 100 mV
S
OUT pp
OPA835
OPA2835
www.ti.com SLOS713E –JANUARY 2011–REVISED JULY 2013
Figure 74. MFB and Sallen-Key 2
nd
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 V
RMS
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 kΩ 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
R
L
= 300 Ω.
Figure 77 and Figure 78 show FFT output with a 1 kHz tone and 100kΩ 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 kΩ load, Figure 77, the curves are basically indistinguishable from each
other except for noise, which means the OPA835 cannot be directly measured. With 300Ω 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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