A Preamp-ADC Interface Amplifier For The BaBar Drift Chamber

The amplifier described in this note is intended to interface an existing preamplifier chip under development by D. Dorfan and N. Spencer at UCSC with a conventional ADC module. The preamp chip provides differential-voltage-mode outputs capable of driving 50-Ω loads. However, the differential channels are made up of emitter-followers with approximately 4Vbe of voltage difference under zero signal conditions. Thus, to first approximation one side of the output will have a quiescent level near 4 volts and a negative signal swing. The other output will have a quiescent level near 1 volt and a positive signal swing. Both channels should be used together to preserve as much power-supply noise rejection as possible. Several conventional ADC chips can be used to provide the dE/dx measurement for the Drift Chamber. The useful dynamic range of the data has been estimated by D. Coupal to be approximately six bits, while a sample rate of 15 MHz in view of the several-hundred-nsec drift time. Conventional ADC units such as the eight-bit Analog Devices AD775 will easily meet these requirements. A six-bit device under development by LBL is also considered a potential candidate. Both of these devices are single-ended. The input signal range of the LBL FADC part is 1-4 volts and is expected to require an interface to the preamp with a gain of two. The full-scale range of the AD775 is two volts peak to peak and will probably not require any gain in the interface. The final gain selection of the preamp-ADC interface amplifier has not been made, but for the purposes of this report a gain of two will be assumed. Finally, since the ADC data may be subject to pile up, an ac-coupled interface should be avoided.