CN-0269

Circuit Note

Rev. 0 | Page 8 of 12

Figure 13. AD8065 Noise Model

The noise sources shown in Figure 13 must be converted to the

output by multiplying the noise gain, which is 1 for a unity-gain

buffer.

2

2

2

2

2

2

2

2

)

(

p

f

p

S

P

V

Rf

RS

RP

i

R

i

R

R

e

e

e

e

+

+

+

+

+

+

The noise from resistors can be calculated from the equation:

Hz

nV/

1000

4

R

e

R

×

=

at 25°C

where R is in Ω.

The Noise of the AD8475Attenuation Stage

AD8475 stage. This noise is reflected to the output of the

AD8475 by multiplying the signal gain (0.4) of AD8475 stage as

shown in Figure 14.

Figure 14. AD8475 Noise Model

The AD8475 output voltage noise is also 10 nV/√Hz, including

amplifier voltage and current noise, as well as noise of internal

resistors.

The noise density of the whole signal chain in front of ADC is

(

)

2

2

_

2

AD8475_RTO

AD8475_RTO

AD8475

GAIN

TOTAL

e

e

GAIN

e

+

×

×

=

The total output noise of the AD8475 is applied to the RC filter

(10 Ω, 2.2 nF) that has a bandwidth of 7.23 MHz. The

bandwidth of the AD8065 is 145 MHz, and the bandwidth of

the AD8475 is 150 MHz. The input bandwidth of the AD7984

ADC is 10 MHz, therefore the noise at the input of the AD7984

is limited by the RC noise filter to 7.23 MHz.

When the AD8475 is operating at a gain of 0.8 (worst case noise

condition) the input rms noise to the ADC is therefore

μV

51

MHz

23

.

7

57

.

1

Hz

nV/

(15.1

_

=

×

×

=

RMS

TOTAL

V

For the 18-bit AD7984 with reference voltage of 4.096 V, the

differential input span is 8.196 V. The LSB value is 31 µV. The

peak-to-peak noise of 337 µV therefore corresponds to 11 LSBs

peak-to-peak.

Effect of Multiplexer Switching Transients

The multiplexer has source and drain capacitance. The drain

capacitance of the multiplexer holds the voltage of previous

input channel. When the multiplexer switches to the next

channel, this can create a transient or kick-back glitch through

Therefore, the pre-filter driver needs to have a very low output

impedance and a fast settling time to the transient.

Figure 15. Multiplexer Switching Transients

The driver needs to be able to charge the input to the required

accuracy (forward-charge) before the switch opens. The back-

charge occurs when the switch opens, and generally is short and

doesn’t present a problem.

In order to make the circuit easy to drive, a buffer can be placed

in front of the multiplexer (front buffer). The evaluation board

EVAL-CN0269-SDPZ has footprints for the input buffer on

each input channel and has an AD8065 installed in Channel 1

to Channel 4. Adding the buffer slightly increases the noise

density and the settling time. However, in a practical application,

the parasitic inductance and capacitance from the input cable

or terminal connector will significantly increase the time of

settling time and generate ringing due to the forward and back

charge without the buffer. The additional input buffer isolates

the parasitic effects and provides very low impedance to the

multiplexer. The difference in performance between the circuit

with or without input buffer is shown in the test part of this

circuit note.

Another reason for adding the input buffer is for that an

additional filter can be placed ahead of it for anti-aliasing and

noise reduction.

AI

AD8065

+

–

+

–

D

SW1

SW2

ADG5208

ADG5236

AI 1

AI 2

PRE-FILTER

+10V

–10V

P1

P2

P3

P1

P2

P3

SW_A0

KICK

BACK-CHARGE

FORWARD-CHARGE