EUA6412
Application Information
Gain Setting
The gain of the EUA6412 is set by two input terminals,
Gain0 and Gain1.The gains listed in Table 1 are realized
by changing the taps on the input resistors inside the
amplifier.
Input Capacitor, Ci
In the typical application an input capacitor, Ci, is
required to allow the amplifier to bias the input signal to
the proper dc level for optimum operation. In this case, Ci
and the input impedance of the amplifier, Zi, from a
high-pass filter with the corner frequency determined in
equation 2.
1
Table 1.Gain Setting
f
=
-----------------(2)
c(highpass)
2π Z C
i
i
GAIN0 GAIN1
ZI
AV(inv)
SE/BTL
0
0
0
1
0
0
0
0
1
6dB
90kΩ
70kΩ
45kΩ
25kΩ
10dB
1
0
15.6dB
21.6dB
4.1dB
1
1
X
X
The value of Ci is important to consider as it directly
affects the bass (low frequency) performance of the
circuit. Consider the example where Zi is 710kΩ and the
specification calls for a flat bass response down to 40Hz.
Equation 2 is reconfigured as equation 3.
Input Resistance
Each gain setting is achieved by varying the input
resistance of the amplifier, which can range from its
smallest value to over 6 times that value. As a results, if a
single capacitor is used in the input high pass filter,
the –3 dB or cut off frequency will also change by over 6
times. If an additional resistor is connected from the input
pin of the amplifier to ground, as shown in the figure
below, the variation of the cut-off frequency will be much
reduced.
1
Ci =
----------------------------- (3 )
2π Z fC
i
In this example, Ci is 5.6nF so one would likely choose a
value in the range of 5.6nF to 1µF. A further
consideration for this capacitor is the leakage path from
the input source through the input network (Ci) and the
feedback network to the load. This leakage current
creates a dc offset voltage at the input to the amplifier
that reduces useful headroom, especially in high gain
applications. For this reason a low- leakage tantalum or
ceramic capacitor is the best choice. When polarized
capacitors are used, the positive side of the capacitor
should face the amplifier input in most applications as
the dc level there is held at VDD/2, which is likely higher
than the source dc level. Note that it is important to
confirm the capacitor polarity in the application.
The-3dB frequency can be calculated using
equation 1:
1
f-3dB
=
---------------------- (1)
2π C (R || R )
i
If the filter must be more accurate, the value of the
capacitor should be increased while the value of the
resistor to ground should be decreased. In addition, the
order of the filter could be increased.
DS6412 Ver 1.0 Dec. 2006
13
EUTECH [ EUTECH MICROELECTRONICS INC ]
