Linear Technology LTC2370-16 with LTC6655-5/LT6202: 16-Bit, 2Msps, Pseudo-Differential Unipolar SAR ADC with 94dB SNR. Req DC718 or DC590 DC1813A-A Fiche De Données

Codes de produits

DC1813A-A

LTC2370-16

237016fa

APPLICATIONS INFORMATION

INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high im-

pedance input of the LTC2370-16 without gain error. A

high impedance source should be buffered to minimize

settling time during acquisition and to optimize the dis-

tortion performance of the ADC. Minimizing settling time

is important even for DC inputs, because the ADC input

draws a current spike when entering acquisition.
For best performance, a buffer amplifier should be used

to drive the analog input of the LTC2370-16. The ampli-

fier provides low output impedance, which produces fast

settling of the analog signal during the acquisition phase.

It also provides isolation between the signal source and

the current spike the ADC input draws.

Input Filtering
The noise and distortion of the buffer amplifier and signal

source must be considered since they add to the ADC noise

and distortion. Noisy input signals should be filtered prior

to the buffer amplifier input with an appropriate filter to

minimize noise. The simple 1-pole RC lowpass filter (LPF1)

shown in Figure 4 is sufficient for many applications.

High quality capacitors and resistors should be used in the

RC filters since these components can add distortion. NPO

and silver mica type dielectric capacitors have excellent

linearity. Carbon surface mount resistors can generate

distortion from self heating and from damage that may

occur during soldering. Metal film surface mount resistors

are much less susceptible to both problems.

Pseudo-Differential Unipolar Inputs
For most applications, we recommend the low power

LT6202 ADC driver to drive the LTC2370-16. With a low

noise density of 1.9nV/√Hz and a low supply current of

3mA, the LT6202 is flexible and may be configured to

convert signals of various amplitudes to the 0V to 5V input

range of the LTC2370-16.
To achieve the full distortion performance of the

LTC2370-16, a low distortion single-ended signal source

driven through the LT6202 configured as a unity-gain buf-

fer as shown in Figure 4 can be used to get the full data

sheet THD specification of –112dB.
The LT6202 can also be used to buffer and convert large

true bipolar signals which swing below ground to the 0V

to 5V input range of the LTC2370-16. Figure 5a shows the

LT6202 being used to convert a ±10V true bipolar signal

for use by the LTC2370-16. In this case, the LT6202 is

configured as an inverting amplifier stage, which acts to

attenuate and level shift the input signal to the 0V to 5V input

range of the LTC2370-16. In the inverting configuration, the

single-ended input signal source no longer directly drives

a high impedance input. The input impedance is instead

set by resistor R

. R

must be chosen carefully based on

the source impedance of the signal source. Higher values

of R

tend to degrade both the noise and distortion of

the LT6202 and LTC2370-16 as a system. Table 1 shows

the resulting SNR and THD for several values of R

, R1,

R2, R3 and R4 in this configuration. Figure 5b shows the

resulting FFT when using the LT6202 as shown in Figure 5a.

Figure 4. Input Signal Chain

Another filter network consisting of LPF2 should be used

between the buffer and ADC input to both minimize the

noise contribution of the buffer and to help minimize distur-

bances reflected into the buffer from sampling transients.

Long RC time constants at the analog inputs will slow

down the settling of the analog inputs. Therefore, LPF2

requires a wider bandwidth than LPF1. A buffer amplifier

with a low noise density must be selected to minimize

degradation of the SNR.

5.1Ω

10nF

66nF

50Ω

LPF2

LPF1

BW = 3.2MHz

BW = 48kHz

LTC2370-16

–

237016 F04

–

LT6202

REF