Applications

Input

As mentioned above, input and output signals are

currents, not voltages. While this often causes some

conceptual difficulty for designers first exposed to this

convention, the current input/output mode provides

great flexibility in application.

The input pin (pin 1) is a virtual ground with nega-

tive feedback provided internally (see Figure 6, Page 4).

The input resistor (shown as 20 k

Ω in Figure 3, Page 3)

should be scaled to convert the available ac input volt-

age to a current within the linear range of the device.

(Peak input currents should be kept under 1 mA for

best distortion performance.) An additional consider-

ation is stability: the internal op amp is intended for

operation with source impedances of less than 30 k

Ω

at high frequencies. For most audio applications, this

will present no problem.

The choice of input resistor has an additional, sub-

tle effect on distortion. Since the feedback impedances

around the internal opamp (essentially Q1/D1 and

Q3/D3) are fixed, low values for the input resistor will

require more closed-loop gain from the opamp. Since

the open-loop gain naturally falls off at high frequen-

cies, asking for too much gain will lead to increased

high-frequency distortion. For best results, this resis-

tor should be kept to 10 k

Ω or above. Distortion vs. fre-

quency for a 1 V signal at 0 dB gain with a 20 k

Ω input

resistor is plotted in Figure 13.

The quiescent dc voltage level at the input is ap-

proximately +10 mV. As mentioned above, any dc input

currents will cause dc signals in the output which will

be modulated by gain, causing audible thump. There-

Figure 10. 1kHz THD+Noise Vs. Input, -15 dB Gain

Figure 11. 1kHz THD+Noise Vs. Level, 0 dB Gain

Figure 12. 1kHz THD+Noise Vs. Input, +15 dB Gain

Figure 13. THD Vs. Frequency, 0 dB Gain

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Rev. 10/25/96

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