5

8/1/00

SPT7936

Data

N-1

Data

N

Data

N+1

Data

Analog Input

Clock

N+2

tH

S

A

M

P

L

E

tAP

S

A

M

P

L

E

N

S

A

M

P

L

E

N+1

tD

GENERAL DESCRIPTION

The SPT7936 is a low power, 12-bit, 28 MSPS ADC. It has a

pipelined architecture and incorporates digital error correc-

tion of the 11 most significant bits. This error correction

ensures good linearity performance for input frequencies up

to Nyquist. The inputs are fully differential, making the device

insensitive to system-level noise. This device can also be

used in a single-ended mode. (See analog input section.)

With the power dissipation roughly proportional to the sam-

pling rate, this device is ideal for very low power applications

in the range of 1 to 28 MSPS.

TYPICAL INTERFACE CIRCUIT

The SPT7936 requires few external components to achieve

the stated operation and performance. Figure 2 shows the

typical interface requirements when using the SPT7936 in

normal circuit operation. The following sections provide a descrip-

tion of the functions and outline critical performance criteria to

consider for achieving the optimal device performance.

Figure 1 - Timing Diagram

Figure 2 - Typical Interface Circuit

D8

D7

D6

D5

D4

D3

D2

D1

D0

VDD

2

D9

VIN-

VIN+

GND

CM

Bias1

Bias0

GND

ExtRef

N/C

GND

Mini-Circuit

T1-6T

VIN

+3.3 V

+1.0 V

+2.0 V

1 k

Ω

1 k

Ω

1 k

Ω

+

+

4.7

µF

.01

µF

+

4.7

µF

.01

µF

+3.3 V

+3.3 V

.01

µF

(MSB)

(LSB)

Clock Input

(3.3 V Logic)

+3.3 V

FB

Bias1

Bias0

ExtRef

50

Ω

68 pF

.01

µF

SPT7936

(*)

NOTES:

1) Place the ferrite bead (*) as close to the device as possible.

2) Place 0.01 microfarad capacitors as close to the device as possible.

3) All capacitors are surface-mount unless otherwise specified.

4) All input pins (references, analog input, clock input) must be

protected. (See absolute maximum rating.)

5) Set Bias1 and Bias0 for maximum sample rate.

Bias1Bias0

0

0

Sleep mode

0

1

Max. 5 MHz sampling

1

0

Max. 20 MHz sampling

1

1

Max. 28 MHz sampling

6) Use internal or external reference. Do not connect external voltage reference when using internal references.

7) All VDD1 and VDD2 must be connected together. Do not leave any pin unconnected.

8) All GND must be connected together. Do not leave any pin unconnected.

4.7

µF

.01

µF

.01

µF

BGAP