REV. 0

AD9845A

–19–

A/D Converter

The AD9845A uses high-performance ADC architecture, opti-

mized for high speed and low power. Differential Nonlinearity

(DNL) performance is typically better than 0.5 LSB, as shown in

TPC 2. Instead of the 1 V full-scale range used by the earlier

AD9801 and AD9803 products from Analog Devices, the

AD9845A’s ADC uses a 2 V input range. Better noise perfor-

mance results from using a larger ADC full-scale range (see

TPC 3).

AUX1 Mode

For applications that do not require CDS, the AD9845A can be

configured to sample ac-coupled waveforms. Figure 30 shows

the circuit configuration for using the AUX1 channel input

(Pin 36). A single 0.1 µF ac-coupling capacitor is needed between

the input signal driver and the AUX1IN pin. An on-chip dc-bias

circuit sets the average value of the input signal to approximately

0.4 V, which is referenced to the midscale code of the ADC.

The VGA Gain register provides a gain range of 0 dB to 36 dB in

this mode of operation (see VGA Gain Curve, Figure 29).

The VGA gains up the signal level with respect to the 0.4 V bias

level. Signal levels above the bias level will be further increased

to a higher ADC code, while signal levels below the bias level

will be further decreased to a lower ADC code.

AUX2 Mode

For sampling video-type waveforms, such as NTSC and PAL

signals, the AUX2 channel provides black level clamping, gain

adjustment, and A/D conversion. Figure 31 shows the circuit

configuration for using the AUX2 channel input (Pin 34). A

external 0.1 µF blocking capacitor is used with the on-chip video

clamp circuit, to level-shift the input signal to a desired refer-

ence level. The clamp circuit automatically senses the most

negative portion of the input signal, and adjusts the voltage

across the input capacitor. This forces the black level of the

input signal to be equal to the value programmed into the Clamp

Level register (see Serial Interface Register Description). The VGA

provides gain adjustment from 0 dB to 18 dB. The same VGA

Gain register is used, but only the 9 MSBs of the gain register

are used (see Table VII.)

AUX1IN

0.1 F

VGA GAIN

REGISTER

ADC

VGA

10

5k

0.4V

0.4V

INPUT SIGNAL

??V

0.8V

0.4V

MIDSCALE

0dB TO 36dB

Figure 30. AUX1 Circuit Configuration

0dB TO 18dB

8

AUX2IN

BUFFER

0.1 F

VIDEO

SIGNAL

9

CLAMP LEVEL

LPF

VGA GAIN

REGISTER

ADC

VGA

VIDEO CLAMP

CIRCUIT

CLAMP LEVEL

REGISTER

Figure 31. AUX2 Circuit Configuration

Table VII. VGA Gain Register Used for AUX2-Mode

MSB

LSB

D10

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

Gain (dB)

X0

XXXXXXXXX0.0

10000000000.0

••

••

••

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