THS4513 datasheet(22/31 Pages) TI

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(

)

EXT

(2)

OUT-

S+

G=1

SIGNAL

S-

OUT+

BIAS = VCM

Wideband

THS4513

Buffer

VS+

VS–

50kW

ToInternal

CMCircuit

EXT

50kW

(

)

(3)

Single-Supply Operation (3 V to 5 V)

THS4513-SP

SLOS539A – SEPTEMBER 2007 – REVISED OCTOBER 2007

performance, 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 63. THS4513 DC Coupled Single-Supply

with Input Biased to VCM

In Figure 64 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 62. CM Input Circuit

VIC is the desired 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

was taken with split-supply power inputs. The device

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

easily can be used with a single-supply power input

values with RPU assuming a 50 Ω source impedance

without

degrading

the

performance.

Figure

63,

and setting the input and output common-mode

Figure 64, and Figure 65 show DC and AC-coupled

voltage to mid-supply.

single-supply circuits with single-ended inputs. These

There are two drawbacks to this configuration. One is

configurations

all

allow

the

input

and

output

that it requires additional current from the power

common-mode voltage to be set to mid-supply

supply. Using the values shown for a gain of 10 dB

allowing for optimum performance. The information

requires 37 mA more current with 5 V supply, and 22

presented here also can be applied to differential

mA more current with 3 V supply.

input sources.

The

other

drawback

this

configuration

also

In Figure 63, the signal source is referenced to a

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

voltage derived from the CM pin via a unity-gain

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

wideband buffer such as the BUF602. VCM is set to

mid-supply by THS4513 internal circuitry. RT along

Table 3. RPU Values for Various Gains

with the input impedance of the amplifier provides

Gain

RIT

RPU

input termination, which also is referenced to VCM.

6 dB

348

Ω

169

Ω

64.9

Ω

200

Ω

Note that RS and RT are added to the alternate input

10 dB

348

Ω

102

Ω

78.7

Ω

133

Ω

from the signal input to balance the amplifier.

Alternately, one resistor can be used equal to the

14 dB

348

Ω

61.9

Ω

115

Ω

97.6

Ω

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

20 dB

348

Ω

40.2

Ω

221

Ω

80.6

Ω

true of the circuits shown in Figure 64 and Figure 65.

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