OPA656 datasheet(13/35 Pages) TI1

13 / 35 page

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at 290K

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THS4631

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SLOS451B

– DECEMBER 2004 – REVISED AUGUST 2011

SUMMARY OF KEY DECISIONS IN

feedback resistors this large or anticipate using an

TRANSIMPEDANCE DESIGN

external compensation scheme to stabilize the circuit.

Using a simple capacitor in parallel with the feedback

The following is a simplified process for basic

resistor makes the amplifier more stable as shown in

transimpedance circuit design. This process gives a

the Typical Characteristics graphs.

start to the design process, though it does ignore

some aspects that may be critical to the circuit.

NOISE ANALYSIS

STEP 1: Determine the capacitance of the source.

High slew rate, unity gain stable, voltage-feedback

STEP 2: Calculate the total source capacitance,

operational amplifiers usually achieve their slew rate

including the amplifier input capacitance, CI(CM)

at the expense of a higher input noise voltage. The

and CI(DIFF).

7 nV/

√Hz input voltage noise for the THS4631 is,

however, much lower than comparable amplifiers

STEP 3: Determine the magnitude of the possible

while achieving high slew rates. The input-referred

current output from the source, including the

voltage noise, and the input-referred current noise

minimum signal current anticipated and

term, combine to give low output noise under a wide

maximum signal current anticipated.

variety of operating conditions. Figure 42 shows the

amplifier noise analysis model with all the noise terms

STEP 4: Choose a feedback resistor value such that

included. In this model, all noise terms are taken to

the input current levels create the desired

be noise voltage or current density terms in either

output signal voltages, and

nV/

√Hz or fA/√Hz.

ensure that the output voltages can

accommodate the dynamic range of the input

signal.

STEP 5: Calculate the optimum feedback

capacitance using Equation 1.

STEP 6: Calculate the bandwidth given the

resulting component values.

STEP 7: Evaluate the circuit to determine if all design

goals are satisfied.

SELECTION OF FEEDBACK RESISTORS

Feedback resistor selection can have a significant

Figure 42. Noise Analysis Model

effect on the performance of the THS4631 in a given

application, especially in configurations with low

closed-loop gain. If the amplifier is configured for

The total output noise voltage can be computed as

unity gain, the output should be directly connected to

the square root of all square output noise voltage

the inverting input. Any resistance between these two

contributors. Equation 7 shows the general form for

points interacts with the input capacitance of the

the output noise voltage using the terms shown in

amplifier and causes an additional pole in the

Figure 42.

frequency response. For nonunity gain configurations,

low resistances are desirable for flat frequency

response. However, care must be taken not to load

the amplifier too heavily with the feedback network if

(7)

large output signals are expected. In most cases, a

Dividing this expression by the noise gain [NG = (1+

trade off is made between the frequency response

Rf/Rg)] gives the equivalent input-referred spot noise

characteristics and the loading of the amplifier. For a

voltage at the noninverting input, as shown in

gain of 2, a 499-

Ωfeedback resistor is a suitable

Equation 8:

operating point from both perspectives. If resistor

values are chosen too large, the THS4631 is subject

to oscillation problems. For example, an inverting

amplifier configuration with a 5-k

Ω gain resistor and a

5-k

Ω feedback resistor develops an oscillation due to

(8)

the interaction of the large resistors with the input

Using high resistor values can dominate the total

capacitance.

low

gain

configurations,

avoid

equivalent

input-referred

noise.

Using

3-k

Ω

source-resistance (RS) value adds a voltage noise

term of approximately 7 nV/

√Hz. This is equivalent to

the amplifier voltage noise term. Using higher resistor

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