ADS8664 Datasheet, PDF (24/79 Pages) Texas Instruments – 4- and 8-Channel, Single-Supply, SAR ADCs

ADS8664, ADS8668
SBAS492 – JULY 2015
www.ti.com
Feature Description (continued)
8.3.3 Input Overvoltage Protection Circuit
The ADS8664 and ADS8668 feature an internal overvoltage protection circuit on each of the four or eight analog
input channels, respectively. Use these protection circuits as a secondary protection scheme to protect the
device. Using external protection devices against surges, electrostatic discharge (ESD), and electrical fast
transient (EFT) conditions is highly recommended. The conceptual block diagram of the internal overvoltage
protection (OVP) circuit is shown in Figure 68.
AVDD
VP+
R_FB
0V
ESD
AVDD
VP- _RS
AIN_nP
R_S
AIN_nGND
10Ÿ _D1p
D2p
AVDD
D1n
V±
V+
+
10Ÿ
D2n
ESD
R_DC
VOUT
V_B
GND
Figure 68. Input Overvoltage Protection Circuit Schematic
As shown in Figure 68, the combination of the 1-MΩ input resistors along with the PGA gain-setting resistors
(R_FB and R_DC) limit the current flowing into the input pins. A combination of antiparallel diodes (D1 and D2) are
added on each input pin to protect the internal circuitry and set the overvoltage protection limits.
Table 1 explains the various operating conditions for the device when the device is powered on. Table 1
indicates that when the AVDD pin of the device is connected to the proper supply voltage (AVDD = 5 V) or offers
a low impedance of < 30 kΩ, the internal overvoltage protection circuit can withstand up to ±20 V on the analog
input pins.
Table 1. Input Overvoltage Protection Limits When AVDD = 5 V or Offers a Low Impedance of < 30 kΩ⁽¹⁾
INPUT CONDITION
(V_OVP = ±20 V)
|V_IN| < |V_RANGE|
Within operating range
|V_RANGE| < |V_IN| < |V_OVP|
Beyond operating range but
within overvoltage range
|V_IN| > |V_OVP|
Beyond overvoltage range
TEST
ADC
CONDITION OUTPUT
COMMENTS
All input
ranges
Valid Device functions as per data sheet specifications
All input
ranges
Saturated
ADC output is saturated, but device is internally
protected (not recommended for extended time)
All input
ranges
Saturated
This usage condition may cause irreversible damage
to the device
(1) GND = 0, AIN_nGND = 0 V, |V_RANGE| is the maximum input voltage for any selected input range, and |V_OVP| is the break-down voltage
for the internal OVP circuit. Assume that R_S is approximately 0.
The results indicated in Table 1 are based on an assumption that the analog input pins are driven by very low
impedance sources (R_S is approximately 0). However, if the sources driving the inputs have higher impedance,
the current flowing through the protection diodes reduces further, thereby increasing the OVP voltage range.
Note that higher source impedance results in gain errors and contributes to overall system noise performance.
24
Submit Documentation Feedback
Product Folder Links: ADS8664 ADS8668
Copyright © 2015, Texas Instruments Incorporated