AD9801

–9–

REV. 0

The actual implementation of this loop is slightly more compli-

cated as shown in Figure 18. Because there are two separate

CDS blocks, two black level feedback loops are required and

two offset voltages are developed. Figure 18 also shows an

additional PGA block in the feedback loop labeled “RPGA.”

PGA

ADC

IN

CLPOB

NEG REF

CONTROL

CDS1

RPGA2

INT2

CDS1

RPGA1

INT1

Figure 18.

The RPGA uses the same control inputs as the PGA, but has

the inverse gain. The RPGA functions to attenuate by the same

factor as the PGA amplifies, keeping the gain and bandwidth of

the loop constant.

Input Bias Level Clamping

The buffered CCD output is connected to the AD9801 through

an external coupling capacitor. The dc bias point for this coupling

capacitor is established during the clamping (CLPDM

= LOW)

period using the “dummy clamp” loop shown in Figure 19.

When closed around the CDS, this loop establishes the desired

DC bias point on the coupling capacitor.

BLACK

LEVEL CLP

CCD

INPUT

CLAMP

CDS

CLPDM

TO ADC

PGA

Figure 19.

Input Blanking

In some applications, the AD9801’s input may be exposed to

large signals from the CCD. These signals can be very large,

relative to the AD9801’s input range, and could thus saturate

on-chip circuit blocks. Recovery time from such saturation

conditions could be substantial.

To avoid problems associated with processing these transients,

the AD9801 includes an input blanking function. When active

(PBLK = LOW), this function stops the CDS operation and

allows the user to disconnect the CDS inputs from the CCD

buffer.

If the input voltage exceeds the supply rail by more than

0.3 V, protection diodes will be turned on, increasing current

flow into the AD9801 (see Equivalent Input Circuits). Such

voltage levels should be externally clamped to prevent device

damage or reliability degradation.

10-Bit Analog-to-Digital Converter (ADC)

The ADC employs a multibit pipelined architecture, which is

well-suited for high throughput rates while being both area and

power efficient. The multistep pipeline presents a low input

capacitance resulting in lower on-chip drive requirements. A

fully differential implementation was used to overcome head-

room constraints of the single +3 V power supply.

Differential Reference

The AD9801 includes a 0.5 V reference based on a differential,

continuous-time bandgap cell. Use of an external bypass

capacitor reduces the reference drive requirements, thus

lowering the power dissipation. The differential architecture was

chosen for its ability to reject supply and substrate noise.

Recommended decoupling shown in Figure 20.

VRT

REF

VRB

1µF

0.1µF

0.1µF

Figure 20.

Internal Timing

The AD9801’s on-chip timing circuitry generates all clocks

necessary for operation of the CDS and ADC blocks. The user

needs only to synchronize the SHP and SHD clocks with the

CCD waveform, as all other timing is handled internally. The

ADCCLK signal is used to strobe the output data, and can be

adjusted to accommodate desired timing.