Modulator Characteristics Highpass character for noise transfer function: In-band noise power is given by
Implementation Switched capacitor circuits - easy to build in a digital CMOS process
- gains and time constants decided by capacitor ratios and clock frequency
Fully differential circuits achieve better power supply rejection and common mode noise rejection Analog characteristics are very sensitive to layout - layouts are made symmetrical to overcome variations in process
Influence of Circuit Parameters - Finite DC gain (“integrator leakage”) causes DC offset and increased baseband noise
- Always build the best possible op-amp for the first integrator
Non-linearity in the feedback D/A converter - Harmonic distortion in the output signal
- Possible modulation of the reference voltage (bad!!)
- A simple 2 level D/A (two switches and a reference voltage) is used
Circuit noise is usually the performance limiting factor - kT/C noise in the capacitors
- kTR noise in the resistors and switches
- Thermal and 1/f noise in the MOSFETS
Example Implementation
Decimation Sample rate conversion from a high rate to Nyquist rate Performed using cascaded digital FIR filters One class of filters used are called CICs (cascaded integrator comb filters) with the transfer function
Sigma Delta D/A Converters Modulator loop is digital Theory and math applicable exactly: quantization error is replaced by truncation error Analog part: A 1 bit D/A followed by one or more filters - Harder to build than A/D counterparts (!!) (analog part has no feedback loop to take advantage of)
- Switched capacitor D/As, Current steering D/As are popular
- Switched capacitor filters followed by a continuous time smoothing filter
- Tapped delay line FIR filters are also used (tends to be larger in area)
Keep analog and digital circuitry on separate power supplies and spaced as far as possible Use the biggest capacitors possible (area and loading on amplifiers are issues) Use the smallest switches possible (lower noise, lower parasitic capacitive coupling) Low thermal and 1/f noise in op-amps Keep signal level as large as possible in the signal path Keep the reference voltage clean (easier said than done!!)
Dostları ilə paylaş: |