Continuous-time delta-sigma analog digital converter having operational amplifiers
The invention concerns a continuous-time delta-sigma analog-digital converter for the conversion of an analog input signal into a digital output signal, comprising an analog filter which filters the analog input signal and at least one externally circuited operational amplifier (OPAMP) for the formation of an integrator stage, a clock-driven quantizer, which quantizes the filtered analog signal outputted through the analog filter to generate the digital output signal, and a feedback arrangement with at least one digital-analog converter, which supplies to the analog filter at least one feedback signal on the basis of the digital output signal. For the reduction of the necessary amplification-bandwidth product for the operational amplifier (OPAMP) it is stipulated according to the invention that the operational amplifier (OPAMP) has a first amplifier path (gm 3 ) and parallel to this a second amplifier path (gm 2 , gm 5 ), wherein the transit frequency of the second amplifier path (gm 2 , gm 5 ) is lower than the transit frequency of the first amplifier path (gm 3 ).
1. A continuous-time delta-sigma analog-digital converter ( 10 ) for the conversion of an analog input signal (Vin) into a digital output signal (Vout), comprising:
an analog filter ( 20 ), which filters the analog input signal and has at least one externally circuited (R, C) operational amplifier (OPAMP 1 , OPAMP 2 , OPAMP 3 ) for the formation of an integration stage ( 22 - 1 , 22 - 2 , 22 - 3 ),
a clock-driven quantiser ( 30 ), which quantises the filtered analog signal outputted by the analog filter ( 20 ) for the generation of the digital output signal, and
a feedback arrangement ( 40 ) with at least one digital-analog converter (DAC 1 , DAC 2 , DAC 3 ), which supplies to the analog filter ( 20 ) at least one analog feedback signal on the basis of the digital output signal (Vout),
characterised in that the operational amplifier (OPAMP 1 , OPAMP 2 , OPAMP 3 ) has a first amplifier path (gm 3 ) and parallel to this a second amplifier path (gm 2 , gm 5 ), wherein the transit frequency of the second amplifier path (gm 2 , gm 5 ) is lower than the transit frequency of the first amplifier path (gm 3 ).
2. The converter according to claim 1 , wherein the transit frequency of the first amplifier path (gm 3 ) is higher than the transit frequency of the integrator stage ( 22 - 1 , 22 - 2 , 22 - 3 ) formed by means of the operational amplifier (OPAMP 1 , OPAMP 2 , OPAMP 3 ).
3. The converter according to claim 1 , wherein for frequencies that are smaller than the transit frequency of the second amplifier path (gm 2 , gm 5 ) the gradient of the amplification of the second amplifier path (gm 2 , gm 5 ) plotted in a Bode diagram is greater by a factor of at least 2 than the corresponding gradient for the first amplifier path (gm 3 ).
4. The converter according to claim 1 , wherein the second amplifier path (gm 2 , gm 5 ) is formed from a connection in series of a plurality of amplifier stages.
5. The converter according to claim 1 , wherein the operational amplifier (OPAMP 1 , OPAMP 2 , OPAMP 3 ) has a third amplifier path (gm 1 , gm 4 , gm 5 ) whose transit frequency is lower than the transit frequency of the second amplifier path (gm 2 , gm 5 ).
6. The converter according to claim 5 , wherein for frequencies that are smaller than the transit frequency of the third amplifier path (gm 1 , gm 4 , gm 5 ) the gradient of the amplification of the third amplifier path (gm 1 , gm 4 , gm 5 ) plotted in a Bode diagram is greater by a factor of at least 2 than the corresponding gradient for the second amplifier path (gm 2 , gm 5 ).
7. The converter according to claim 5 , wherein the third amplifier path (gm 1 , gm 4 , gm 5 ) is formed from a connection in series of a number of amplifier stages, this number being greater than the number of amplifier stages of the second amplifier path (gm 2 , gm 5 ).
8. The converter according to claim 1 , wherein the quantiser ( 30 ) has a plurality of quantising stages.
9. The converter according to claim 1 , wherein the digital output signal (Vout) of the quantiser ( 30 ) possesses a thermometer code.