THS4513 datasheet(26/34 Pages) TI

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OUT-

S+

G=1

SIGNAL

S-

OUT+

BIAS

= V

Wideband

THS4513

Buffer

S+

–

50 kW

to internal

CM circuit

EXT

50 kW

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Single-Supply Operation (3V to 5V)

THS4513

SLOS472 – AUGUST 2005

formance, but it is intended to be a DC bias input pin.

Bypass capacitors are recommended on this pin to

reduce noise at the output. The external current

required to overdrive the internal resistor divider is

given by Equation 2:

where VCM is the voltage applied to the CM pin.

Figure 79. THS4513 DC Coupled Single-Supply

with Input Biased to VCM

In Figure 80 the source is referenced to ground and

so is the input termination resistor. RPU is added to

the circuit to avoid violating the VICR of the op amp.

The proper value of resistor to add can be calculated

from Equation 3:

Figure 78. CM Input Circuit

VIC is the desire input common-mode voltage, VCM =

To facilitate testing with common lab equipment, the

CM, and RIN = RG+ RS||RT. To set to mid-supply,

THS4513 EVM allows split-supply operation, and the

make the value of RPU = RG+ RS||RT.

characterization data presented in this data sheet

Table 3 is a modification of Table 1 to add the proper

was taken with split-supply power inputs. The device

values with RPU assuming a 50 Ω source impedance

can easily be used with a single-supply power input

and setting the input and output common-mode

without degrading the performance. Figure 79, Fig-

voltage to mid-supply.

ure 80, and Figure 81 show DC and AC-coupled

single-supply circuits with single-ended inputs. These

There are two drawbacks to this configuration. One is

configurations all allow the input and output com-

it requires additional current from the power supply.

mon-mode voltage to be set to mid-supply allowing

Using the values shown for a gain of 0 dB requires 14

for optimum performance. The information presented

mA more current with 5 V supply, and 8.2 mA more

here can also be applied to differential input sources.

current with 3 V supply.

In Figure 79, the signal source is referenced to a

The other drawback is this configuration also in-

voltage derived from the CM pin via a unity-gain

creases the noise gain of the circuit. In the 10 dB

wideband buffer such as the BUF602. VCM is set to

gain case, noise gain increases by a factor of 1.5.

mid-supply by THS4513 internal circuitry. RT along

with the input impedance of the amplifier provides

Table 3. RPU Values for Various Gains

input termination, which is also referenced to VCM.

Gain

RIT

RPU

Note that RS and RT are added to the alternate input

0 dB

348

Ω

340

Ω

56.2

Ω

365

Ω

from the signal input to balance the amplifier. Alter-

6 dB

348

Ω

168

Ω

64.9

Ω

200

Ω

nately, one resistor can be used equal to the com-

bined value RG+ RS||RT on this input. This is also true

of the circuits shown in Figure 80 and Figure 81.

THS4513 Datasheet(PDF) 26 Page - Texas Instruments

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