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Texas Instruments ADS16xx Reference Options Application notes
Application Report
SLAA717 – August 2016
ADS16xx Reference Options
Tom Hendrick......................................................................... Precision Analog – Delta Sigma Converters
ABSTRACT
This application note presents methods for applying the internal reference or external reference options for
use with the 16-bit ADS1602 and ADS1605 and the 18-bit ADS1625 and ADS1626.
1
Introduction
The ADS1605/06 and ADS1626/26 can operate from an internal or external voltage reference. In either
case, the reference voltage VREF is set by the differential voltage between VREFN and VREFP: VREF =
(VREFP – VREFN). VREFP and VREFN each use two pins, which should be shorted together. VMID
equals approximately 2.5 V and is used by the modulator. VCAP connects to an internal node and must
also be bypassed with an external capacitor. For the best analog performance, it is recommended that an
external reference voltage (VREF) of 3 V be used.
2
Internal Reference
To use the internal reference, set the REFEN pin low. This activates the internal circuitry that generates
the reference voltages. The internal reference voltages are applied to the pins. Good bypassing of the
reference pins is critical to achieve optimum performance and is done by placing the bypass capacitors as
close to the pins as possible. Figure 1 shows the recommended bypass capacitor values. Use high quality
ceramic capacitors for the smaller values. Avoid loading the internal reference with external circuitry. If the
ADS1605/6 or ADS1625/26 internal reference is to be used by other circuitry, buffer the reference
voltages to prevent directly loading the reference pins.
ADS1605/6
ADS1625/6
0.1 mF
10mF
VREFP
VREFP
22mF
22mF
VMID
0.1 mF
0.1mF
10mF
22mF
VREFN
VREFN
10mF
0.1mF
VCAP
0.1mF
AGND
Figure 1. Internal Reference Bypassing
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ADS16xx Reference Options
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1
Internal Reference
2.1
www.ti.com
Internal Reference Startup
The internal bandgap reference has two startup mechanisms. The first is a static mechanism that ensures
some current is present in the output stage of the bandgap. The second is a transient mechanism which
ensures the proper output voltage. This dual startup approach provides coverage across silicon process
variations and is triggered at power-up of the DVDD supply or by toggling the power down (PD) pin.
When not in use, the ADS1605/06 and ADS1625/26 can be powered down by taking the PD pin low. All
circuitry will be shutdown, including the voltage reference. There is an internal pull-up resistor of 170 kΩ
on the PD pin, but it is recommended that this pin be connected to a general purpose I/O pin to help
ensure proper startup of the reference when power cycling the device at cold temperatures.
When power cycling the ADS1605/06 and ADS1625/26 below - 20°C, it is recommended to allow the
voltage present on the VCAP pin (pin 59) to drain completely to 0 V before the application of the analog
supply voltage (AVDD) and the digital supply voltage (DVDD). If the reference voltage does not recover at
power-up, toggling the /PD pin will allow the reference to recover.
3
External Reference
To use an external reference, set the REFEN pin high. This deactivates the internal generators for
VREFP, VREFN and VMID, and saves approximately 25 mA of current on the analog supply (AVDD). The
voltages applied to these pins must be within the values specified in the Electrical Characteristics table.
Typically VREFP = 4 V, VMID = 2.5 V and VREFN = 1 V. The external circuitry must be capable of
providing both a dc and a transient current.
Figure 2 shows the recommended circuitry for driving these reference inputs. Keep the resistances used
in the buffer circuits low to prevent excessive thermal noise from degrading performance. Layout of these
circuits is critical, make sure to follow good high-speed layout practices. Place the buffers and especially
the bypass capacitors as close to the pins as possible. VCAP is unaffected by the setting on REFEN and
must be bypassed when using the internal or an external reference.
392W
0.001mF
ADS1605/6
ADS1625/6
VREFP
VREFP
OPA2822
10mF
4V
0.1mF
392W
0.1mF
0.001mF
22µF
22µF
VMID
OPA2822
10mF
2.5V
0.1mF
392W
0.001mF
22µF
VREFN
VREFN
OPA2822
1V
10mF
0.1mF
VCAP
0.1mF
AGND
Figure 2. Recommended External Reference Buffer Circuit
2
ADS16xx Reference Options
SLAA717 – August 2016
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