Circuit arrangement and method for sigma-delta conversion with reduced idle tones
The circuit arrangement has a sigma-delta converter ( 2 ) for converting an analog input signal into a digital output signal. The sigma-delta converter ( 2 ) contains a loop filter, a comparator connected downstream of the latter and a feedback loop to feed the output signal back to the input signal. To reduce idle tones a dither signal is fed to the comparator by means of a dither-signal line ( 27.1 ). The dither signal is not however generated by a complex dither-signal generator. Instead, what is used as a dither signal is a signal that is available in the circuit but is not specifically generated for this purpose, e.g. an output signal from a second sigma-delta converter ( 2 ′). The circuit arrangement is thus simpler and less expensive than conventional circuit arrangements without its reduction of idle tones suffering.
1. A circuit arrangement having a sigma-delta converter ( 2 ) for convening an analog input signal into a digital output signal, wherein the sigma-delta converter ( 2 ) comprises a loop filter ( 22 ) having a filter input to which an input line ( 21 ) for the input signal is connected and having a filter output, a quantizer ( 23 ) having a quantizer input that is connected to the filter output and having a quantizer output to which an output line ( 24 ) for the output signal is connected, and a feedback loop ( 25 ) to feed the output signal back to the input signal, and the circuit arrangement has a dither-signal line ( 27 ) that is suitable for additionally applying to the quantizer input, as a dither signal, a signal tat is available in the circuit without a separate dither generator.
2. A circuit arrangement as claimed in claim 1 , wherein the circuit comprises a second sigma-delta converter ( 2 ′) having a second output line ( 24 ′), and the dither-signal line ( 27 . 1 ) connects the second output line ( 24 ′) to the quantizer input of the first sigma-delta converter ( 2 ).
3. A circuit arrangement as claimed in claim 1 , wherein the circuit arrangement comprises a digital-to-analog converter ( 5 ), preferably an FIR digital-to-analog converter, and the dither-signal line ( 27 . 3 ) connects an input line of the digital-t-analog converter ( 5 ) to the quantizer input of the first sigma-delta convener ( 2 ).
4. A circuit arrangement as claimed in claim 1 , wherein the circuit arrangement has means ( 4 ) for noise shaping that are connected upstream of the dither-signal line ( 27 . 3 ).
5. A circuit arrangement as claimed in claim 1 , wherein the quantizer ( 23 ) comprises a comparator.
6. A circuit arrangement as claimed in claim 1 , wherein the feedback loop ( 25 ) comprises a digital-to-analog converter ( 26 ).
7. A method for the sigma-delta conversion of an analog input signal into a digital output signal comprising the following method steps:
(a) filtering of the input signal, thereby producing a filtered signal;
(b) adding together of the filtered signal and a dither signal, thereby producing a sum signal, what is used as the dither signal being a signal that is available in the circuit but is not specifically generated for this purpose;
(c) quantizing of the sum signal, thereby producing the output signal; and
(d) feeding-back of the output signal to the input signal.
8. A method as claimed in claim 7 , wherein an output signal obtained by the sigma-delta conversion of a different input signal is used as the dither signal.
9. A method as claimed in claim 7 , wherein an input signal to a digital-to-analog converter ( 5 ), preferably an FIR digital-to-analog converter, is used as the dither signal.
10. A method as claimed in claim 7 , wherein a signal containing wide-band noise, preferably a noise-shaped signal, is used as the dither signal.