THS3201D datasheet(16/29 Pages) TI

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THS3201

SLOS416A − JUNE 2003 − REVISED JANUARY 2004

www.ti.com

THS3201

47pF

ADC

47pF

1:n

24.9

Ω

0.1

µF

VS−

VS+

24.9

Ω

0.1

µF

0.1

µF

VIN

Figure 55. Differential ADC Driver Circuit 2

It is almost universally recommended to use a resistor and

capacitor between the op amp’s output and the ADC’s

input as shown in both Figures.

This resistor-capacitor (RC) combination has multiple

functions:

D The capacitor is a local charge reservoir for ADC

D The resistor isolates the amplifier from the ADC

D In conjunction, they form a low-pass noise filter

During the sampling phase, current is required to charge

the ADC’s input sampling capacitors. By placing external

capacitors directly at the input pins, most of the current is

drawn from them. They are seen as a very low impedance

source. They can be thought of as serving much the same

purpose as a power supply bypass capacitor; to supply

transient current, with the amplifier then providing the bulk

charge.

Typically, a low-value capacitor in the range of 10 pF to

100 pF provides the required transient charge reservoir.

The capacitance and the switching action of the ADC is

one of the worst loading scenarios that a high-speed

amplifier encounters. The resistor provides a simple

means of isolating the associated phase shift from the

feedback network and maintaining the phase margin of the

amplifier.

Typically, a low value resistor in the range of 10

Ω to 100 Ω

provides the required isolation. Together, the R and C form

real

pole

the

s-plane

located

the

frequency:

P +

pRC

Placing this pole at about 10x the highest frequency of

interest insures it has no impact on the signal. Since the

resistor is typically a small value, it is very bad practice to

place the pole at (or very near) frequencies of interest. At

the pole frequency, the amplifiers sees a load with a

magnitude of:

2 xR

If R is only 10

Ω, the amplifier is very heavily loaded above

the pole frequency, and generates excessive distortion.

DAC DRIVER APPLICATION

The THS3201 can be used as a high-performance DAC

output driver in applications like radio transmitter stages,

and arbitrary waveform generators. All high-performance

DACs have differential current outputs. Two THS3201s

can be used as a differential drive amplifier in these

applications as shown in Figure 56.

RPU on the DAC output is used to convert the output

current to voltage. The 24.9-

Ω resistor and 47-pF capacitor

between each DAC output and the op amp input is used

to reduce the images generated at multiples of the

sampling rate. The values shown form a pole a 136 MHz.

ROUT sets the output impedance of each amplifier.

IOUT2

DAC

IOUT1

THS3201

47pF

24.9

Ω

VS+

0.1

µF

ROUT

VOUT1

VOUT2

ROUT

VS−

0.1

µF

24.9

Ω

AVDD

RPU

0.1

µF

Figure 56. Differential DAC Driver Circuit

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