ADA4351-2ACPZ-R7 datasheet(32/36 Pages) AD

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Data Sheet

ADA4351-2

APPLICATIONS INFORMATION

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Rev. 0 | 32 of 36

ERROR BUDGET

The output offset error contributed by the ADA4351-2 in a transi-

mpedance application consists of three major sources: IOFF, IB,

and VOS. In addition, there are error contributions from CMRR and

PSRR, although these errors can be reduced by using accurate

supplies and calibration. The gain error of the transimpedance

amplifier is the tolerance of the feedback resistance.

For a 5 V nominal supply voltage, calculate the full output error by

using the following equation:

Output Error (V)=VOS+ IOFF+IB− RF +

IB+ RIN+ +10−CMRR20VS2−VCM+

10−PSRR20 5−VS1−∆VS100 + ID ∆RF (6)

where:

IB− is the input bias current at the inverting input.

IB+ is the input bias current at the noninverting input.

RIN+ is the source resistance at the noninverting input.

CMRR and PSRR are in dB.

ΔVS is the highest the supply can be in the application minus the

lowest the supply can be in the application.

ΔRF is the percent tolerance × 100 of RF.

If the noninverting resistance is kept at a minimum, the IB− × RIN+

is insignificant. The CMRR term is reduced by operating at VCM

equal to midsupply; however, this may not be suitable for many

applications. An initial calibration can alleviate the error related to

CMRR. The error contributed by PSRR can be reduced by using

accurate supplies or by an initial calibration. The gain error can be

reduced by using more accurate feedback resistors.

It is also useful to look at the input-referred error at different values

of feedback resistance to define a given TIA application. Figure 93

shows the input-referred percent error for transimpedance values of

5 kΩ and 500 kΩ.

Figure 93. DC Input Referred Error vs. Input Current (ID)

At a low ID and a low RF, the offset voltage dominates the input

error. At a low ID and a high RF, the bias and leakage currents

dominate the input error. The input error reduces with an increased

current level until the amplifier clips and error shoots up.

ADC DRIVING

The ADA4351-2 can be used for directly driving a successive

approximation register (SAR) ADC. The slew rate and THD of the

ADA4351-2 contribute to the low distortion even at larger output

levels. The ADA4351-2 also draws low supply current and can thus

be paired with low power, high resolution ADCs.

Selecting the External RC Filter Components

Figure 94 shows a typical single-supply application using the

AD4696, a high accuracy, low power, 16-channel, 16-bit SAR ADC.

The ADA4351-2 is configured as an ADC driver that can switch

between gains. At the output of the ADA4351-2, which is also at the

analog front end of the ADC, is an external low-pass filter formed by

REXT and CEXT. Note that these components reduce the wideband

noise and nonlinear voltage kickback in the analog inputs of the

ADCs.

Figure 94. Typical Single-Supply Application Using the AD4696

The selection for the RC filter is an iterative process, and the best

combination depends on the intended application. For example,

in lower frequency applications, the designer can opt to reduce

the corner frequency by choosing a higher value RC to introduce

less noise, but the designer must also make sure that this RC

combination allows the signal to settle faster than the selected

acquisition phase duration of the ADC.

For detailed information on selecting an RC filter configuration, see

the Analog Devices, Inc., Analog Dialogue article, Front-End Ampli-

fier and RC Filter Design for a Precision SAR Analog-to-Digital

Converter. In addition, refer to the ADC data sheet when selecting

these components as well.

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