SGM89000 Datasheet, PDF (9/14 Pages) SG Micro Corp – Capless 2Vrms Line Driver

SGM89000
Capless 2Vrms Line Driver
with Adjustable Gain
APPLICATION INFORMATION (continued)
Input-Blocking Capacitors
DC input-blocking capacitors are required to be added in series with the audio signal into the input pins of the
SGM89000. These capacitors block the DC portion of the audio source and allow the SGM89000 inputs to be
properly biased to provide maximum performance. The input blocking capacitors also limit the DC-gain to one,
limiting the DC-offset voltage at the output.
These capacitors form a high-pass filter with the input resistor, R_IN. The cutoff frequency is calculated using
Equation 1. For this calculation, the capacitance used is the input-blocking capacitor and the resistance is the input
resistor chosen from Table 1, then the frequency and/or capacitance can be determined when one of the two values
are given.
1
1
fc _IN = _2πRINCIN or C_IN = _{2πfc R IN IN} (1)
Using the SGM89000 as 2nd-Order Filter
Several audio DACs used today require an external low-pass filter to remove out of band noise. This is possible with
the SGM89000 as it can be used like a standard OPAMP.
Several filter topologies can be implemented both single-endedly and differentially. In Figure 4, a Multi Feedback
(MFB), with differential input and single-ended input is shown.
An AC coupling capacitor to remove DC-content from the source is shown. It serves to block any DC-content from
the source and lowers the DC-gain to one, helping reducing the output DC-offset to minimum.
C_IN
R_IN
R1
-IN
C2
+IN
C_IN
R_IN
R1
R_FB
C1
_
+
C1
C_IN
R_IN
R1
-IN
C2
R_FB
C1
_
+
R_FB
Figure 4. 2nd-Order Active Low Pass Filter
The resistor values should be kept low to obtain low noise, but should also be high enough to get a small size AC
coupling cap. Using 5.6kΩ for the resistors, C1 = 220pF, and C2 = 470pF, an SNR of 114dB can be achieved with a
10μF input AC coupling capacitor.
Pop-Free Power-Up
Pop-free power-up is ensured by keeping the SD (EN) (shutdown pin) low during power supply ramp up and down.
The EN pin should be kept low until the input AC coupling capacitors are fully charged before asserting the EN pin
high. This way proper precharge of the AC coupling is performed, and pop-free power-up is achieved. Figure 5
illustrates the preferred sequence.
SG Micro Corp
www.sg-micro.com
FEBRUARY 2017
9