ADAQ7980 datasheet(29/49 Pages) AD

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Data Sheet

ADAQ7980/ADAQ7988

Rev. 0 | Page 29 of 49

Figure 58 illustrates the primary noise contributors for the

typical gain configurations. The total output noise (vn_out) is the

root sum square of all the noise contributions.

in–

4kTRS

vn_RS =

4kTRG

vn_RG =

vn_RF =

+ vn_out –

4kTRF

in+

Figure 58. Noise Sources in Typical Connection

Calculate the output noise spectral density of the ADC driver by



out

kTR

kTRs

kTR



































where:

k is the Boltzmann constant.

T is the absolute temperature in degrees Kelvin.

RF and RG are the feedback network resistances, as shown in

Figure 58.

RS is the source resistance, as shown in Figure 58.

in+ and in− represent the amplifier input current noise spectral

density in pA/√Hz.

vn is the amplifier input voltage noise spectral density in

nV/√Hz.

For more information on these calculations, see MT-049 and

MT-050.

Source resistance noise, amplifier input voltage noise (vn), and

the voltage noise from the amplifier input current noise

(in+ × RS) are all subject to the noise gain term (1 + RF/RG).

Figure 59 shows the total referred to input (RTI) noise due

to the amplifier vs. the source resistance. Note that with a

5.9 nV/√Hz input voltage noise and 0.6 pA/√Hz input current

noise, the noise contributions of the amplifier are relatively

small for source resistances from approximately 2.6 kΩ to 47 kΩ.

The Analog Devices, Inc., silicon germanium (SiGe) bipolar

process makes it possible to achieve a low voltage noise. This

noise is much improved compared to similar low power amplifiers

with a supply current in the range of hundreds of microamperes.

100

10k

100k

SOURCE RESISTANCE (Ω)

TOTAL NOISE

SOURCE RESISTANCE NOISE

AMPLIFIER NOISE

SOURCE RESISTANCE = 2.6kΩ

SOURCE RESISTANCE = 47kΩ

Figure 59. RTI Noise vs. Source Resistance

Keep the noise generated by the driver amplifier, and its

associated passive components, as low as possible to preserve

the SNR and transition noise performance of the ADAQ7980/

ADAQ7988. The analog input circuit of the ADAQ7980/

ADAQ7988 features a one-pole, low-pass filter to band limit the

noise coming from the ADC driver. Because the typical noise of

the ADAQ7980/ADAQ7988 is 44.4 μV rms in the dual-supply

typical configuration, the SNR degradation due to the amplifier is



















)

(

44.4

log

LOSS

SNR

where:

f–3 dB is the cutoff frequency of the input filter (4.4 MHz).

N is the noise gain of the amplifier (for example, 1 in a buffer

configuration).

eN is the equivalent input noise voltage of the op amp, in

nV/√Hz.

For multichannel multiplexed applications, the analog input

circuit of the ADAQ7980/ADAQ7988 must settle a full-scale

step onto the capacitor array at a 16-bit level (0.0015%, 15 ppm)

within one conversion period. As shown in Figure 20, the

bandwidth of the ADC driver changes with the gain setting

implemented. The ADC driver must maintain a sufficient

bandwidth to allow the ADC input to settle properly. The RC

time constant of the low-pass filter of the ADAQ7980/ADAQ7988

has been set to settle the anticipated SAR ADC charge redistribution

voltage step from a full-scale ADC input voltage transition within

the minimum acquisition phase of the ADC. The maximum

full-scale step is based upon the maximum reference input

voltage of 5.1 V. The reference sets the maximum analog input

range and subsequently the range of voltages that the ADC can

quantize.

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