LTC2302 datasheet(14/20 Pages) LINER

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LTC2302/LTC2306

23026fa

APPLICATIONS INFORMATION

Figure 2b. Analog Input Equivalent Circuit for

Large Filter Capacitances

Figure 2a. Analog Input Equivalent Circuit

chargingthesample-and-holdcapacitorsduringtheacquire

mode. In conversion mode, the analog inputs draw only

a small leakage current. If the source impedance of the

driving circuit is low, the ADC inputs can be driven directly.

Otherwise, more acquisition time should be allowed for a

source with higher impedance.

Reference

A low noise, stable reference is required to ensure full

performance. The LT®1790 and LT6660 are adequate

for most applications. The LT6660 is available in 2.5V,

3V, 3.3V and 5V versions, and the LT1790 is available in

1.25V, 2.048V, 2.5V, 3V, 3.3V, 4.096V and 5V versions.

The exceptionally low input noise allows the input range to

be optimized for the application by changing the reference

voltage. The VREF input must be decoupled with a 10μF

capacitor in parallel with a 0.1μF capacitor, so verify that

the device providing the reference voltage is stable with

capacitive loads.

If the voltage reference is 5V and can supply 5mA, it can

be used for both VREF and VDD. VDD must be connected

to a clean analog supply, and a quiet 5V reference voltage

makes a convenient supply for this purpose.

Input Filtering

The noise and distortion of the input ampliﬁer and other

circuitry must be considered since they will add to the

ADC noise and distortion. Therefore, noisy input circuitry

should be ﬁltered prior to the analog inputs to minimize

noise. A simple 1-pole RC ﬁlter is sufﬁcient for many

applications.

The analog inputs of the LTC2302/LTC2306 can be modeled

as a 55pF capacitor (CIN) in series with a 100Ω resistor

(RON) as shown in Figure 2a. CIN gets switched to the

selected input once during each conversion. Large ﬁlter

RC time constants will slow the settling of the inputs. It

is important that the overall RC time constants be short

enough to allow the analog inputs to completely settle to

12-bit resolution within the acquisition time (tACQ) if DC

accuracy is important.

When using a ﬁlter with a large CFILTER value (e.g., 1μF),

the inputs do not completely settle and the capacitive input

switching currents are averaged into a net DC current

(IDC). In this case, the analog input can be modeled by

an equivalent resistance (REQ = 1/(fSMPL • CIN)) in series

with an ideal voltage source (VREF/2) as shown in Figure

2b. The magnitude of the DC current is then approximately

IDC = (VIN – VREF/2)/REQ, which is roughly proportional

to VIN. To prevent large DC drops across the resistor

RFILTER, a ﬁlter with a small resistor and large capacitor

should be chosen. When running at the minimum cycle

time of 2μs, the input current equals 106μA at VIN = 5V,

which amounts to a full-scale error of 0.5LSB when using

a ﬁlter resistor (RFILTER) of 4.7Ω. Applications requiring

lower sample rates can tolerate a larger ﬁlter resistor for

the same amount of full-scale error.

CIN

55pF

RON

100Ω

RSOURCE

VIN

LTC2302

LTC2306

INPUT

(CH0, CH1

IN+, IN–)

23026 F02a

REQ

1/(fSMPL • CIN)

VREF/2

RFILTER

IDC

VIN

LTC2302

LTC2306

INPUT

(CH0, CH1

IN+, IN–)

CFILTER

23026 F02b

–

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