Linear Technology has introduced the dual LTC2158-14 and single LTC2153-14 high IF sampling 14bit, 310Msample/s ADCs for wide bandwidth DPD (digital predistortion) linearisation applications. DPD is a closed-loop feedback system that samples the distortion bandwidth at the output of the base station transmitter and adjusts the input signal to cancel the power amplifier's intermodulation distortion products. This enables the transmitter to operate at 1dB compression point, where the power amplifier response is nonlinear.
The transmit bandwidths are 20 to 40MHz; depending on whether an IF sampling or I/Q sampling DPD architecture was implemented. To linearise a transmit bandwidth of 20MHz, the feedback loop for the linearisation algorithm must acquire fifth order intermodulation products out to 100MHz, requiring a 12bit ADC with a minimum sample rate of 200Msamples/s for IF sampling, or 100Msamples/s for I/Q sampling.
The dual IF enables linearisation of transmission bandwidths up to 60MHz using I/Q sampling, and offers a short pipeline latency of five clock cycles for fast adaptation. The single IF sampling architectures have transmit bandwidths of up to 30MHz.
Operating from a single 1.8V supply, the dual IF sampling ADC consumes 362mW per channel at 310Msamples/s and offers SNR performance of 68.8dB and spurious free dynamic range of 88dB at baseband with an easy-to-drive 1.32Vpp input range. The IFs are part of a pin-compatible of 170 to 310Msample/s dual and single ADCs, offered in 14 and 12bit resolutions.
Analogue full power bandwidth of 1.25GHz and low jitter of 0.15/s RMS enables undersampling of IF frequencies with good noise performance. The ADCs offer double data rate LVDS digital outputs as well as programmable LVDS output current and optional 100Ω termination.
