Circuit Note

CN-0385

Rev. 0 | Page 3 of 13

The AD8475 funnel amplifier provides high precision attenuation

(0.4×), accurate common-mode level shifting, and single-ended

to differential conversion. Its low output noise spectral density

(10 nV/√Hz) and fast settling time (50 ns to 0.001% for a 2 V

output step) make it well suited to drive the AD4003.

The AD4003 is a fully-differential, 2 MSPS, 18-bit precision SAR

ADC that features a typical signal-to-noise ratio (SNR) of 98 dB

when using a 4.096 V reference. The AD4003 is also low power,

and only consumes approximately 17 mW at full throughput. Its

power consumption scales with throughput, and can operate at

lower sample rates to cut its power use (for example, 0.17 mW

at 100 kSPS).

System DC Accuracy Errors

Figure 2 shows the ideal transfer function of the data acquisition

system.

Figure 2. ADC Ideal Transfer Function

Each of the components in the data acquisition signal chain

adds its own offset error and gain error that cause the real

transfer function of the system to deviate from the ideal transfer

function shown in Figure 2. The cumulative effects of these

errors can be measured at a system level by comparing known

dc inputs near zero and full scale at the input to the ADG5207

(RC filter if it is present) and the resulting output codes from

the AD4003 to obtain a system calibration factor.

Offset Error Measurement

For ideal bipolar, differential ADCs, a 0 V differential input

results in an output code of 0. Real ADCs typically exhibit some

ideal output code and the measured output code for a 0 V input.

The offset error for the data acquisition system can be found by

grounding its input and observing the resulting output code. This

error varies between each of the gain settings of the AD8251 and

between each of the channels of the ADG5207. Offset error is

therefore measured for each of the channels in all four gain

configurations.

Because the system monitors multiple channels, it is also important

to quantify the amount by which the offset error deviates between

imum deviation between the offset error of each of the channels

and the average offset error of all of the channels. Offset error

match is calculated using the following equation:

)

7

...,

,1

,

0

|

)

8

(max(

7

0

,

,

,

=

ε

−

ε

=

ε

∆

∑

=

i

j

j

b

i

b

MAX

b

respectively.

This offset error match can be found for each of the gain

configurations. Note that offset error can be expressed either in

codes or volts.

Gain Error Measurement

Error in the gain of the system also contributes to overall system

inaccuracy. The ideal transfer function of the AD4003 is shown in

a negative full-scale input voltage (−FS) and a positive full-scale

input voltage (+FS), respectively; however, the combination of

this relationship.

Gain error can be expressed as a percentage error between the

actual system gain and the ideal system gain. The more common

expression is in percent full-scale error (%FS), which is a measure

of the error between the ideal and actual input voltages that

resolution of the ADC (18-bits for the AD4003) and the

reference translate to gain errors in the ADC. To decouple

precision multimeter. The ideal full-scale input voltage can then

be calculated using

MEAS

REF

MEAS

REF

IDEAL

FS

V

V

V

,

17

,

18

,

2

2

2

=

×

=

The actual system gain can be found by calculating the slope of

and the resulting output codes:

using

LR

LR

REAL

REAL

FS

m

m

Y

V

17

,

2

=

=

The gain error (expressed in %FS error) can then be calculated

using

%

100

,

,

,

×

−

=

ε

IDEAL

FS

REAL

FS

IDEAL

FS

m

V

V

V

The gain error of the system varies with the gain of the AD8251,

but is channel independent. Therefore, gain error is measured

for each of the four gain configurations, but only using one of

the ADG5207 channels in this system.

100...000

100...001

100...010

011...101

011...110

011...111

ANALOG INPUT

+FSR – 1.5 LSB

+FSR – 1 LSB

–FSR + 1 LSB

–FSR

–FSR + 0.5 LSB