Differential Preamp

Stephen Kaye

2013-3-22

New Preamp Design Motivation

• Move preamplifier from controller to the Vacuum Interface Board (VIB)

• Amplifies signal near the CCDs– Larger signal less

susceptible to noise during transmission

• Use dummy CCD output to create differential transmission– Decrease susceptibility to

noise and crosstalk by creating and transmitting a differential signal

Preamp Design

DifferentialInput from CCD

AC Coupled

DC Black Level Clamp

Preamp Design (Generic)

Preamp Design - Gain

• Image area full well is 275,000e- (Adjust clocks not to exceed this.)

• 16 bit ADC differential input voltage range is +- 4.096V (AD7625)

• Map this to 60,000 ADU, so conversion gain = 4.6e-/ADU

• CCD sensitivity = 7 µV/e-

• Total video gain = 1.95 V/V

• All video gain is in preamp stage

Preamp Design – Black Level Clamp

• Switch connects AC coupling capacitors to ground

• Switch activated during vertical transfers

• Transients from switch are coupled to both inputs and are common mode

Preamp Design - Noise

Preamp Design - NoiseInput Referred Voltage Noise

CCD Source Follower

40 nV/√Hz

Source Follower Load Resistor

4.7 nV/√Hz

Op-Amp Feedback Resistor

1.4 nV/√Hz

Op-Amp Bridge Resistor

4 nV/√Hz

Op-Amp Input Voltage Noise

2.8 nV/√Hz

Op-Amp Input Current Noise

1.5 nV/√Hz

Clamp Leakage Current Noise

0.125 pV/√Hz

Preamp Design – Output Impedance

• Preamp is driving twinax cables

• Twinax cable impedance is 100 Ω

• Series resistors are 100 Ω for serial termination