diff --git a/src/ad5680.rs b/src/ad5680.rs
index f8b4b77..a20e294 100644
--- a/src/ad5680.rs
+++ b/src/ad5680.rs
@@ -6,7 +6,6 @@ use stm32f4xx_hal::{
     time::MegaHertz,
     spi,
 };
-use crate::units::Volts;
 
 /// SPI Mode 1
 pub const SPI_MODE: spi::Mode = spi::Mode {
@@ -45,9 +44,7 @@ impl<SPI: Transfer<u8>, S: OutputPin> Dac<SPI, S> {
         Ok(())
     }
 
-    pub fn set(&mut self, voltage: Volts) -> Result<(), SPI::Error> {
-        let value = ((voltage.0 * (MAX_VALUE as f64) / 5.0) as u32)
-            .min(MAX_VALUE);
+    pub fn set(&mut self, value: u32) -> Result<(), SPI::Error> {
         let buf = [
             (value >> 14) as u8,
             (value >> 6) as u8,
diff --git a/src/channel.rs b/src/channel.rs
index c0b72a2..fb4022c 100644
--- a/src/channel.rs
+++ b/src/channel.rs
@@ -2,7 +2,6 @@ use crate::{
     ad5680,
     channel_state::ChannelState,
     pins::{ChannelPins, ChannelPinSet},
-    units::Volts,
 };
 
 /// Marker type for the first channel
@@ -16,6 +15,8 @@ pub struct Channel<C: ChannelPins> {
     pub state: ChannelState,
     /// for `i_set`
     pub dac: ad5680::Dac<C::DacSpi, C::DacSync>,
+    /// 1 / Volts
+    pub dac_factor: f64,
     pub shdn: C::Shdn,
     /// stm32f4 integrated adc
     pub adc: C::Adc,
@@ -30,11 +31,13 @@ impl<C: ChannelPins> Channel<C> {
     pub fn new(pins: ChannelPinSet<C>) -> Self {
         let state = ChannelState::default();
         let mut dac = ad5680::Dac::new(pins.dac_spi, pins.dac_sync);
-        let _ = dac.set(Volts(0.0));
+        let _ = dac.set(0);
+        // sensible dummy preset. calibrate_i_set() must be used.
+        let dac_factor = ad5680::MAX_VALUE as f64 / 5.0;
 
         Channel {
             state,
-            dac,
+            dac, dac_factor,
             shdn: pins.shdn,
             adc: pins.adc,
             vref_pin: pins.vref_pin,
diff --git a/src/channels.rs b/src/channels.rs
index 459c5cf..d4e839e 100644
--- a/src/channels.rs
+++ b/src/channels.rs
@@ -1,6 +1,8 @@
 use stm32f4xx_hal::hal::digital::v2::OutputPin;
 use smoltcp::time::Instant;
+use log::info;
 use crate::{
+    ad5680,
     ad7172,
     channel::{Channel, Channel0, Channel1},
     channel_state::ChannelState,
@@ -83,14 +85,21 @@ impl Channels {
 
     /// i_set DAC
     pub fn set_dac(&mut self, channel: usize, voltage: Volts) {
+        let dac_factor = match channel.into() {
+            0 => self.channel0.dac_factor,
+            1 => self.channel1.dac_factor,
+            _ => unreachable!(),
+        };
+        let value = (voltage.0 * dac_factor) as u32;
+        info!("set_dac {} {}", voltage, value);
         match channel {
             0 => {
-                self.channel0.dac.set(voltage).unwrap();
+                self.channel0.dac.set(value).unwrap();
                 self.channel0.state.dac_value = voltage;
                 self.channel0.shdn.set_high().unwrap();
             }
             1 => {
-                self.channel1.dac.set(voltage).unwrap();
+                self.channel1.dac.set(value).unwrap();
                 self.channel1.state.dac_value = voltage;
                 self.channel1.shdn.set_high().unwrap();
             }
@@ -142,6 +151,7 @@ impl Channels {
         }
     }
 
+    /// should be 1.5V
     pub fn read_vref(&mut self, channel: usize) -> Volts {
         match channel {
             0 => {
@@ -185,4 +195,44 @@ impl Channels {
             _ => unreachable!(),
         }
     }
+
+    /// for i_set
+    pub fn calibrate_dac_value(&mut self, channel: usize) {
+        let vref = self.read_vref(channel);
+        let mut best_value = 0;
+        let mut best_error = Volts(100.0);
+        for value in 1..=ad5680::MAX_VALUE {
+            match channel {
+                0 => {
+                    self.channel0.dac.set(value).unwrap();
+                    self.channel0.shdn.set_high().unwrap();
+                }
+                1 => {
+                    self.channel1.dac.set(value).unwrap();
+                    self.channel1.shdn.set_high().unwrap();
+                }
+                _ => unreachable!(),
+            }
+
+            let dac_feedback = self.read_dac_feedback(channel);
+            let error = vref - dac_feedback;
+            if error < Volts(0.0) {
+                info!("calibration done at {} > {}", dac_feedback, vref);
+                break;
+            } else if error < best_error {
+                best_value = value;
+                best_error = error;
+            }
+        }
+
+        self.set_dac(channel, Volts(0.0));
+        info!("best dac value for {}: {}, itec={}", vref, best_value, self.read_itec(channel));
+
+        let dac_factor = best_value as f64 / vref.0;
+        match channel {
+            0 => self.channel0.dac_factor = dac_factor,
+            1 => self.channel1.dac_factor = dac_factor,
+            _ => unreachable!(),
+        }
+    }
 }
diff --git a/src/main.rs b/src/main.rs
index d70b68a..84642c5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -96,6 +96,8 @@ fn main() -> ! {
         dp.ADC1, dp.ADC2, dp.ADC3,
     );
     let mut channels = Channels::new(pins);
+    channels.calibrate_dac_value(0);
+
     timer::setup(cp.SYST, clocks);
 
     #[cfg(not(feature = "generate-hwaddr"))]