REV. B

AD9058

–7–

t

V

t

PD

N

N + 1

ANALOG INPUT

ENCODE

D – D

0

7

DATA

CHANGING

VALID DATA

FOR N – 1

VALID DATA

FOR N

VALID DATA

FOR N + 1

N + 2

PRECEDING ENCODE

t

= APERTURE TIME

Figure 4. AD9058 Timing Diagram

Figure 5 below shows what the analog input to the AD9058 would

look like when observed relative to the sample clock. Signal-

to-noise ratio (SNR), transient response, and sample rate are all

critical specifications in digitizing this “eye pattern.”

ANALOG

INPUT

SAMPLE

CLOCK

Figure 5. AD9058 I and Q Input Signals

Receiver sensitivity is limited by the SNR of the system. For the

ADC, SNR is measured in the frequency domain and calculated

with a Fast Fourier Transform (FFT). The signal-to-noise ratio

equals the ratio of the fundamental component of the signal

(rms amplitude) to the rms level of the noise. Noise is the sum

of all other spectral components, including harmonic distortion,

but excluding dc.

Although the signal being sampled does not have a significant

slew rate at the instant it is encoded, dynamic performance of

the ADC and the system is still critical. Transient response is the

time required for the AD9058 to achieve full accuracy when a

step function input is applied. Overvoltage recovery time is the in-

terval required for the AD9058 to recover to full accuracy after

an overdriven analog input signal is reduced to its input range.

Time domain performance of the ADC is also extremely impor-

tant in digital oscilloscopes. When a track (sample)-and-hold is

used ahead of the ADC, its operation becomes similar to that

described above for receivers.

The dynamic response to high-frequency inputs can be described

by the effective number of bits (ENOB). The effective number of bits

is calculated with a sine wave curve fit and is expressed as:

where N is the resolution (number of bits) and measured error

is actual rms error calculated from the converter’s outputs with

a pure sine wave applied as the input.

Maximum conversion rate is defined as the encode (sample) rate

at which SNR of the lowest frequency analog test signal drops

no more than 3 dB below the guaranteed limit.

60

55

50

45

40

35

INPUT FREQUENCY – MHz

0.1

1

10

100

30

+25°C

–55°C

+125°C

Figure 6. Harmonic Distortion vs. Analog Input Frequency

0.1

1

10

100

INPUT FREQUENCY – MHz

8.0

7.2

6.4

5.5

55

50

45

40

35

30

+25°C AND +125°C

–55°C

Figure 7. AD9058 Dynamic Performance vs. Analog Input

Frequency