SPT7755A データシートの表示（PDF） - Signal Processing Technologies

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SPT7755A

8-BIT, 750 MSPS FLASH A/D CONVERTER

Signal Processing Technologies 

GENERAL DESCRIPTION

The SPT7755 is one of the fastest monolithic 8-bit parallel

flash A/D converters available today. The nominal conver-

sion rate is 750 MSPS and the analog bandwidth is in excess

of 900 MHz. A major advance over previous flash converters

is the inclusion of 256 input preamplifiers between the

reference ladder and input comparators (see block dia-

gram). This not only reduces clock transient kickback to the

input and reference ladder due to a low AC beta but also

reduces the effect of the dynamic state of the input signal on

the latching characteristics of the input comparators. The

preamplifiers act as buffers and stabilize the input capaci-

tance so that it remains constant over different input voltage

and frequency ranges and therefore makes the part easier to

drive than previous flash converters. The preamplifiers also

add a gain of two to the input signal so that each comparator

has a wider overdrive or threshold range to "trip" into or out

of the active state. This gain reduces metastable states that

can cause errors at the output.

The SPT7755 has true differential analog and digital data

paths from the preamplifiers to the output buffers (Current

Mode Logic) for reducing potential missing codes while

rejecting common mode noise.

Signature errors are also reduced by careful layout of the

analog circuitry. The output drive capability of the device can

provide full ECL swings into 50 Ω loads.

Figure 1 - SPT7755 Typical Interface Circuit

VIN**

VIN

50 Ω

VIN

VRTF

R

VRTS

R

-2.0 V

Reference

+

22 Ω

- U1

VRM

*

-

22 Ω 2N2907

+U1

*

VRBS

VRBF

-5.2 V

-5.2 V

DRB (DATA READY)

DRB (DATA READY)

D8B (OVR)

D7B (MSB)

D6B

D5B

D4B

D3B

D2B

D1B

D0B (LSB)

DRA (DATA READY)

DRA (DATA READY)

D8A (OVR)

D7A (MSB)

D6A

D5A

D4A

D3A

D2A

D1A

D0A (LSB)

Convert

U2

50 Ω

50 Ω

CLK

NCLK

-2 V

Pulldown

(Analog)

* FB

-5.2 V

.1 µF

-2.0 V

Pulldown

(Digital)

FB= Ferrite bead

U1= OP291 or equivalent with low offset/noise.

R = 1 k Ω; 0.1% matched.

= AGND

= DGND

U2= Motorola ECLinPS Lite, MC10EL16, differential receiver

with 250 ps (typ) propagation delay.

* = 10 µF Tantalum Capacitor and 0.1 µF Chip Capacitor

** = Care must be taken to avoid exceeding the maximum rating

for the input, especially during power up sequencing of the

analog input driver.

SPT

4

SPT7755

3/5/97

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