Good morning. afternoon. I have a customer considering the use of the **ADA4800 (Low Power, High Speed CCD Buffer Amplifier)** with scientific grade CCDs and he has several questions:

1) The first concern is with the current noise of the current source load. Current noise on the load will develop a noise voltage across the CCD's on-chip amplifier output impedance. That noise will then be buffered by the ADA4800 internal op-amp and show up on the output. **Is there any information on the noise current as none appears to be included in the data sheet? ** The customer believes that it may be somewhat noisier than the "discrete" loads they have been using, but they don't have fully quantified, measured results to share (at least not yet).

2) The second issue is the allowable current that can safely be input to the IN pin during power-up. The absolute maximum ratings only state Vcc is the maximum. **Are there protection doides built in? How much current can they handle safely? **The customer uses CCDs that have output voltages > 20V, but can put a dropping resistor, with a bypass capacitor across it, to reduce the voltage at the input to < Vcc. However, during the power-up cycle, the voltage may go above Vcc until the capacitor is charged. **Do you have any experience or guidelines for this situation? **The customer may be able to control the rise of the CCD Vdd so the capacitor gets to equilibrium, if they know how much current they had available. we had available.

Thank you.

Hello,

Sorry for the late reply.

1. We don't have hard data on the current noise. However you can estimate it as follows:

The test circuit used to characterize this part is shown on Figure 2 of the datasheet. An equivalent circuit for noise analysis could look like this:

Assume the noise density of the buffer is 1.1 nV/rt Hz. This is a reasonable value for these type of high speed amps. With this amp noise density at 27'C temperature the current noise will work out to be 9.6 pA/rt Hz.

Did I underestimate the amp noise? If so, assume it is 1.2 nV/rt Hz. With this value the current noise will work out to be zero.

Did I over estimate the amp noise? If so, assume it is 1 nV/ rt Hz. It would be unreasonable to go any lower for this type of buffer. With this value the current noise will work out to be 12 pA/rtHz.

So, the current noise you are looking for is somewhere in the range of very small to 12 pA/rt Hz.

2. Yes, there are ESD protection diodes. See Figure 24 in the datasheet. These diodes are designed to carry "large" currents, up to about 10 mA but only for very short periods of time.

If customer can put series resistors in the path, size the resistor such that the worst case current will be 5mA or less.

Zoltan