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LM1971MX Datenblatt(PDF) 10 Page - Texas Instruments

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Teilenummer LM1971MX
Bauteilbeschribung  Overture Audio Attenuator Series Digitally Controlled 62 dB Audio Attenuator
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LM1971
SNAS104B – FEBRUARY 1995 – REVISED APRIL 2013
www.ti.com
Attenuation level changes cause changes in the output impedance of a
μPot. Output impedance changes in the
presence of a large input bias current for a buffer/amplifier will cause a DC shift to occur. Neglecting amplifier
gains and speaker sensitivities, the audibility of a DC shift is dependent upon the output impedance change
times the required input bias current. As an example, a 5 k
Ω impedance change times a 1 μA bias current results
in a 5 mV DC shift; a level that is barely audible without any music material in the system. An op amp with a bias
current of 200 pA for the same 5 k
Ω change results in an inaudible 1 μV DC shift. Since the worst case output
impedance changes are on the order of several k
Ω, a bias current much less than 1 μA is required for highest
performance. In order to further quantify DC shifts, please refer to Figure 14 in Typical Performance
Characteristics and relate worst case impedance changes to the selected buffer/amplifier input bias current.
Without the use of a high input impedance (> 1 M
Ω) op amp for the buffer/amplifier, loading will occur that
causes linearity errors in the signal. To ensure the highest level of performance, a JFET or CMOS input high
input impedance op amp is required.
One common application that requires gain at the output of a
μPot is input signal volume control. Depending
upon the input source material, the LM1971 provides a means of controlling the input signal level. With a supply
voltage range of 4.5V to 12V, the LM1971 has the ability of controlling fairly inconsistent input source signal
levels. Using an op amp with gain at the
μPot's output, as shown in Figure 20, will also allow the system dynamic
range to be increased. JFET op amps like the LF351 and the LF411 are well suited for this application. If active
half-supply buffering is also desired, dual op amps like the LF353 and the LF412 could be used.
For low voltage supply applications, op amps like the CMOS LMC6041 are preferred. This part has a supply
operating range from 4.5V–15.5V and also comes in a surface mount package.
μPOT HALF-SUPPLY REFERENCING
The LM1971 operates off of a single supply, with half-supply biasing supplied at the VREFIN terminal (Pin 1). The
easiest and most cost effective method of providing this half-supply is a simple resistor divider and bypass
capacitor network shown in Figure 1. The capacitor not only stabilizes the half-supply node by “holding” the
voltage nearly constant, but also decouples high frequency signals on the supply to ground. Signal feedthrough,
power supply ripple and fluctuations that are not properly filtered could cause the performance of the LM1971 to
be degraded.
A more stable half-supply node can be obtained by actively buffering the resistor divider network with a voltage
follower as shown in Figure 19. Supply fluctuations are then isolated by the high input impedance/low output
impedance mismatch associated with effective filtering. Since the LM1971 is a single channel device, using a
dual JFET input op amp is optimum for both output buffering and half-supply biasing.
A 10
μF capacitor or larger is recommended for better half-supply stabilization. For added rejection of higher
frequency power supply fluctuations, a smaller capacitor (0.01
μF–0.1 μF) could be added in parallel to the 10 μF
capacitor.
Figure 19. Higher Performance
Active Half-Supply Buffering
10
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