SGM8905 Datasheet, PDF (8/13 Pages) SG Micro Corp – Capless 2Vrms to 3Vrms Line Driver

SGM8905
Capless 2Vrms to 3Vrms Line Driver
with Adjustable Gain
APPLICATION INFORMATION
Decoupling Capacitors
The SGM8905 is a capless line driver amplifier that requires adequate power supply decoupling to ensure that the
noise and total harmonic distortion (THD) are low. A good low equivalent-series-resistance (ESR) ceramic capacitor,
typically 1µF, placed as close as possible to the device V_DD lead, works best. Placing this decoupling capacitor close
to the SGM8905 is important for the performance of the amplifier. For filtering lower frequency noise signals, a 10µF
or larger capacitor placed near the audio power amplifier would also help, but it is not required in most applications
because of the high PSRR of this device.
Input-Blocking Capacitors
DC input-blocking capacitors are required to be added in series with the audio signal into the input pins of the
SGM8905. These capacitors block the DC portion of the audio source and allow the SGM8905 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,
then the frequency and/or capacitance can be determined when one of the two values are given.
1
1
fc _IN = _{2πR C IN IN} or C_IN = _{2πfc R IN IN} (1)
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 1
illustrates the preferred sequence.
Supply
Supply Ramp
SD
Time for AC coupling
capacitors to charge
Figure 1. Power-Up Sequence
SG Micro Corp
www.sg-micro.com
MARCH 2017
8