AD585 查看數據表（PDF） - Analog Devices

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AD585

Sample-and-Hold Amplifier High Speed, Precision

Analog Devices 

AD585

For the AD585 in particular it becomes:

S/H

Offset

(V ) =

100

0.3 pC

pF + (CEXT

)

The addition of an external hold capacitor also affects the acqui-

sition time of the AD585. The change in acquisition time with

respect to the CEXT is shown graphically in Figure 2.

HOLD MODE

In the hold mode there are two important specifications that

must be considered; feedthrough and the droop rate. Feedthrough

errors appear as an attenuated version of the input at the output

while in the hold mode. Hold-Mode feedthrough varies with fre-

quency, increasing at higher frequencies. Feedthrough is an im-

portant specification when a sample and hold follows an analog

multiplexer that switches among many different channels.

Hold-mode droop rate is the change in output voltage per unit

of time while in the hold mode. Hold-mode droop originates as

leakage from the hold capacitor, of which the major leakage

current contributors are switch leakage current and bias current.

The rate of voltage change on the capacitor dV/dT is the ratio of

the total leakage current IL to the hold capacitance CH.

Droop

Rate

=

dVOUT

dT

(Volts/Sec) =

IL ( pA)

CH ( pF )

For the AD585 in particular;

Droop

Rate

=

100

100 pA

pF + (CEXT

)

Additionally the leakage current doubles for every 10°C increase

in temperature above 25°C; therefore, the hold-mode droop rate

characteristic will also double in the same fashion. The hold-mode

droop rate can be traded-off with acquisition time to provide the

best combination of droop error and acquisition time. The tradeoff

is easily accomplished by varying the value of CEXT.

Since a sample and hold is used typically in combination with

an A/D converter, then the total droop in the output voltage has

to be less than 1/2 LSB during the period of a conversion. The

maximum allowable signal change on the input of an A/D

converter is:

∆V max =

Full -Scale Voltage

2(N +1)

Once the maximum ∆V is determined then the conversion time

of the A/D converter (TCONV) is required to calculate the maxi-

mum allowable dV/dT.

dV

dt

max

=

∆V max

T CONV

dV max

The maximum dT as shown by the previous equation is

the limit not only at 25°C but at the maximum expected operat-

ing temperature range. Therefore, over the operating temperature

range the following criteria must be met (TOPERATION –25°C)

= ∆T.

(∆T °C)

dV 25° C × 2 10°C ≤ dV max

dT

dT

HOLD-TO-SAMPLE TRANSITION

The Nyquist theorem states that a band-limited signal which is

sampled at a rate at least twice the maximum signal frequency

can be reconstructed without loss of information. This means

that a sampled data system must sample, convert and acquire

the next point at a rate at least twice the signal frequency. Thus

the maximum input frequency is equal to

( ) f MAX

=

2

1

T ACQ + TCONV

+ T AP

Where TACQ is the acquisition time of the sample-to-hold

amplifier, TAP is the maximum aperture time (small enough to

be ignored) and TCONV is the conversion time of the A/D

converter.

DATA ACQUISITION SYSTEMS

The fast acquisition time of the AD585 when used with a high

speed A/D converter allows accurate digitization of high fre-

quency signals and high throughput rates in multichannel data

acquisition systems. The AD585 can be used with a number of

different A/D converters to achieve high throughput rates. Fig-

ures 12 and 13 show the use of an AD585 with the AD578 and

AD574A.

Figure 12. A/D Conversion System, 117.6 kHz Throughput

58.8 kHz max Signal Input

Figure 13. 12-Bit A/D Conversion System, 26.3 kHz

Throughput Rate, 13.1 kHz max Signal Input

REV. A

–5–

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