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AD9058ATJ/883

Dual 8-Bit 50 MSPS A/D Converter

Analog Devices 

AD9058

surface mount packages. To evaluate the AD9058 in through-

hole PCB designs, use the AD9058AJD/AKD with individual pin

sockets (AMP part #6-330808-0). Alternatively, surface mount

AD9058 units can be mounted in a through-hole socket (Circuit

Assembly Corporation, Irvine California part #CA-44SPC-T).

AD9058 APPLICATIONS

Combining two ADCs in a single package is an attractive alter-

native in a variety of systems when cost, reliability and space are

important considerations. Different systems emphasize particu-

lar specifications, depending on how the part is used.

In high density digital radio communications, a pair of high

speed ADCs are used to digitize the in-phase (I) and quadrature

(Q) components of a modulated signal. The signal presented to

each ADC in this type of system consists of message-dependent

amplitudes varying at the symbol rate, which is equal to the

sample rates of the converters.

ANALOG

INPUT

N

tA

N+1

N+2

ENCODE

tV

D0–D7 VALID DATA

FOR N–1

VALID DATA

FOR N

VALID DATA

FOR N+1

tPD

DATA

CHANGING

tA = APERTURE TIME

tV = DATA DELAY OF PRECEDING ENCODE

tPD = OUTPUT PROPAGATION DELAY

Figure 4. AD9058 Timing Diagram

Figure 5 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

interval 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:

ENOB = N – LOG2 [Error (measured)/Error (ideal)]

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

+25 C

50

–55 C

+125 C

45

40

35

30

0.1

1

10

100

INPUT FREQUENCY – MHz

Figure 6. Harmonic Distortion vs. Analog Input Frequency

55

50

8.0

+25 C AND +125 C

45

7.2

40

6.4

–55 C

35

5.5

30

0.1

1

10

100

INPUT FREQUENCY – MHz

Figure 7. AD9058 Dynamic Performance vs. Analog Input

Frequency

–8–

REV. C

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