LTC2326-16_15 Datasheet, PDF (12/24 Pages) Linear Technology – True Bipolar, Pseudo-Differential Input ADC with 93.5dB SNR

LTC2326-16
Applications Information
Table 1. Internal Reference with Internal Buffer
REFIN
REFBUF
BIPOLAR INPUT RANGE
2.048V
4.096V
±10.24V
Table 2. External Reference with Internal Buffer
REFIN
(OVERDRIVE)
REFBUF
BIPOLAR INPUT RANGE
1.25V (Min)
2.5V
±6.25V
2.048V
4.096V
±10.24V
2.4V (Max)
4.8V
±12V
Table 3. External Reference Unbuffered
REFIN
REFBUF
(OVERDRIVE)
BIPOLAR INPUT RANGE
0V
2.5V (Min)
±6.25V
0V
5V (Max)
±12.5V
Internal Reference with Internal Buffer
The LTC2326-16 has an on-chip, low noise, low drift
(20ppm/°C max), temperature compensated bandgap
reference that is factory trimmed to 2.048V. It is internally
connected to a reference buffer as shown in Figure 5a and
is available at REFIN (Pin 8). REFIN should be bypassed to
GND with a 100nF ceramic capacitor to minimize noise. The
reference buffer gains the REFIN voltage by 2 to 4.096V at
REFBUF (Pin 7). So the input range is ±10.24V, as shown
in Table 1. Bypass REFBUF to GND with at least a 47μF
ceramic capacitor (X7R, 10V, 1210 size) to compensate
the reference buffer and minimize noise.
REFIN
100nF
REFBUF
47µF
+
REFERENCE
BUFFER
–
6.5k
15k _BANDGAP
REFERENCE
6.5k
GND
LTC2326-16
232616 F05a
Figure 5a. LTC2326-16 Internal Reference Circuit
External Reference with Internal Buffer
If more accuracy and/or lower drift is desired, REFIN
can be easily overdriven by an external reference since
a 15k resistor is in series with the reference as shown
in Figure 5b. REFIN can be overdriven in the range from
1.25V to 2.4V. The resulting voltage at REFBUF will be
2 • REFIN. So the input range is ±5 • REFIN, as shown
in Table 2. Linear Technology offers a portfolio of high
performance references designed to meet the needs of
many applications. With its small size, low power, and high
accuracy, the LTC6655-2.048 is well suited for use with
the LTC2326-16 when overdriving the internal reference.
The LTC6655-2.048 offers 0.025% (max) initial accuracy
and 2ppm/°C (max) temperature coefficient for high pre-
cision applications. The LTC6655-2.048 is fully specified
over the H-grade temperature range and complements
the extended temperature range of the LTC2326-16 up to
125°C. Bypassing the LTC6655-2.048 with a 2.7μF to 100µF
ceramic capacitor close to the REFIN pin is recommended.
External Reference Unbuffered
The internal reference buffer can also be overdriven from
2.5V to 5V with an external reference at REFBUF as shown
in Figure 5c. So the input ranges are ±6.25V to ±12.5V,
respectively, as shown in Table 3. To do so, REFIN must
be grounded to disable the reference buffer. A 13k resistor
loads the REFBUF pin when the reference buffer is disabled.
To maximize the input signal swing and corresponding
SNR, the LTC6655-5 is recommended when overdriv-
ing REFBUF. The LTC6655-5 offers the same small size,
accuracy, drift and extended temperature range as the
LTC6655-2.048. By using this 5V reference, an SNR of
94.5dB can be achieved. Bypassing the LTC6655-5 with
a 47μF ceramic capacitor (X5R, 0805 size) close to the
REFBUF pin is recommended.
The REFBUF pin of the LTC2326-16 draws a charge (Q_CONV)
from the external bypass capacitor during each conversion
cycle. If the internal reference buffer is overdriven, the
external reference must provide all of this charge with a
DC current equivalent to I_REFBUF = Q_CONV/t_CYC. Thus, the
DC current draw of REFBUF depends on the sampling
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